Castrovalva (TV story)

Castrovalva was the first serial of season 19 of Doctor Who. It was the first full television serial to feature Peter Davison as the Fifth Doctor. Beginning with this story, Doctor Who was moved from its traditional Saturday evening slot on BBC1 to air twice weekly.

This story would also be the final mention of Block Transfer Computation, as it's incredibly difficult in-universe for non-Logopolitans to do so. Even the Master, a genius in his own right, had to find a way to circumvent the difficulty.

As of this story, Doctor Who returned to the plot of the Doctor having three companions along in his travels. However, the writers would later admit that it was hard to write a role for three different companions, which is why either Adric, Nyssa or Tegan would be incapacitated in some way during an adventure.

Synopsis
The Doctor's latest regeneration has proven more unstable than his previous ones. His two companions, Tegan and Nyssa, help him recuperate on the tranquil planet of Castrovalva.

Adric has been captured by the Master, who is taking advantage of the Doctor's weakened state to ensnare him in an elaborate trap whose recursive nature threatens to destroy his mind.

As the Doctor begins to recover he realises that the Master's trap for Adric is even more intricate than he could have imagined and that he will stop at nothing to gain his revenge over the Doctor.

Part one
At the Pharos Project in Cambridgeshire, the Fourth Doctor's companions, Adric, Nyssa and Tegan run to where he has fallen from the radio telescope. The Doctor is dying, but smiles as he looks up at his companions who have gathered at his side and says, "It's the end... but the moment has been prepared for." They turn to see the Watcher approach, and Nyssa realises that "he was the Doctor all the time." As the companions look on, the Watcher merges with the dying Doctor, triggering his fourth regeneration. Moments later the Fifth Doctor sits up, beaming with a delighted grin.

Moments later, wishing to avoid being arrested for trespassing, Tegan, Nyssa and Adric are helping the newly regenerated Doctor to the TARDIS. However, he collapses from his post-regenerative exhaustion. They are all captured by the Pharos Project's security guards. The unconscious Doctor is loaded into an ambulance, while the head of security tells the companions these are secure premises; and orders them to lean on the ambulance so they can be frisked for weapons. Adric tries to convince the sceptical guards that he and his friends are alien intelligences who came in answer to the Pharos Project's call, and points up at the sky — supposedly indicating the way into E-Space. This distracts the guards long enough for Tegan and Nyssa to steal the ambulance and drive it to the TARDIS.

Once inside, the Doctor regains consciousness and bolts into the corridors. Before Tegan and Nyssa can follow him, the Master's TARDIS materialises outside, and bolts of energy strike down Adric and the guards. Tegan and Nyssa emerge from the Doctor's TARDIS just as the Master's vanishes, leaving a stunned Adric kneeling amongst the unconscious bodies of the guards. Tegan and Nyssa usher him into the TARDIS, where he immediately crosses to the console and sets the controls for flight. Without so much as a word of thanks, Adric then sets off to join the Doctor, who is unravelling his old clothing to leave a trail as he searches his ship for its Zero Room. The Doctor's erratic personality is beginning to disintegrate, and he drifts through behavioural patterns of his previous incarnations, calls Adric by the names of former companions - and realises that his regeneration is failing.

While waiting for the Doctor and Adric to return, Tegan and Nyssa discover the TARDIS data bank and try to find its index file. The frustrated Tegan eventually points out that if they had an index file they could look up the index file in the index file under "index file". Nyssa points out that this is a perfect example of the mathematical concept of recursion, and Tegan, musing on the power of the word "if", realises that "I.F." stands for "Index File". They are thus able to access the TARDIS' destination setting and determine that they are on a programmed course and unlikely to crash. But they have no idea what "Hydrogen Inrush: Event One" means.

The oddly unresponsive Adric abandons the Doctor in the corridors and sets off in search of the Zero Room on his own, growing ever more worried and panicky as he does so. The Master, watching from the safety of his own TARDIS, is amused by his behaviour, knowing Adric will never be free until their task is complete. The Doctor, who's been left to his own devices finds a room filled with cricketing equipment and changes into a cricket outfit with a beige coloured trenchcoat and a panama hat which is to be his new attire. He then hears the distant sound of the Zero Room door slamming shut, and rushes off to investigate, running into Tegan and Nyssa on the way. He is continuing to weaken and nearly collapses before Nyssa finds the Zero Room, an environment completely isolated from the rest of the Universe. Once cut off from the complexity of the outside world the Doctor's health is restored, although it will take quite some time for him to fully recover from the stress of his latest regeneration.

The Doctor drifts off to sleep, telling his companions that they will all have their parts to play in helping him to heal and to pilot the TARDIS. But Tegan and Nyssa see Adric pinned against a wall of the Zero Room -- and he warns them that he is, in fact, a Block Transfer Computation and that the real Adric is now a prisoner of the Master. The image of Adric breaks up while warning the girls to check the coordinates; the Master made him set a trap. Nyssa decides to investigate on her own, leaving Tegan to watch over the Doctor while she returns to the console room. On her way back, she notices that the ambient temperature in the corridors is increasing. Meanwhile, the Master muses over the curious fact that the image of Adric had nearly developed a will of its own towards the end.

The Doctor awakens, convinced that something is wrong somewhere - and then the cloister bell begins to sound. He insists upon investigating but as soon as he steps out of the Zero Room he falls as though struck. Tegan orders him to remain in the Zero Room while she finds out what's going on. The Doctor is unable to remain in the Zero Room and do nothing, but when he emerges the TARDIS begins to buck under some sort of turbulence and throws him unconscious to the floor. Tegan arrives in the console room as the temperature continues to increase, to find that Nyssa has used the data bank to research the "Hydrogen Inrush" - and has discovered that Adric programmed the TARDIS to travel back in time to the creation of the universe. As the TARDIS begins to shake with turbulence, the Master appears on the TARDIS' scanner, gloating, as Adric hangs pinned in a web of power lines in the background...

Part two
Nyssa turns off the scanner screen so she won't have to see the Master any longer, and she and Tegan try to find some way of escaping from the time pressure of Event One. The Master, meanwhile, hovers nearby in time and space, using Adric as a source of mathematical calculations to generate Block Transfer Computations. He attempts to visualise the destruction of the Doctor's TARDIS, but Adric's resistance interferes with the reception. The Master suggests that as the Doctor is doomed, Adric might as well join forces with him and co-operate willingly... and after some thought, Adric agrees.

The Doctor regains consciousness as turbulence drops medical supplies from a TARDIS roundel onto his head and sends an electric wheelchair rolling down the corridor towards him. He manages to get to the console room, where Nyssa realises that the neurochemical reactions stimulated by the crisis have focussed his concentration. For the moment at least, he is thinking clearly, and he sends Nyssa to vent the TARDIS' thermal buffers while showing Tegan how to use the Architectural Configuration. If they delete a quarter of the TARDIS' internal structure they will generate enough power and thrust to escape the time pressure of Event One. But as the temperature inside the TARDIS returns to normal the Doctor's body chemistry stabilises -- and he loses consciousness before he can explain how to ensure that the console room is not deleted. Tegan and Nyssa have no choice but to delete a quarter of the TARDIS at random.

The Master watches and gloats as the TARDIS is apparently destroyed in the nexus of Event One. He prepares to release Adric from the power web, as he no longer requires his backup plan -- or so he thinks until he finds residual power in the Hadron lines, revealing that Adric is picking up an image and hiding it from him. He boosts power through the web, burning through Adric's resistance - and revealing that the TARDIS has survived and escaped Event One after all.

Tegan and Nyssa look up information on failed regenerations in the TARDIS data bank, and find a suggestion that Time Lords can relax in Dwellings of Simplicity with little or no technological advancement. The suggested location is Castrovalva, the central habitation of the planet that forms the Phylox series - according to the TARDIS records at this point. Tegan prepares to pilot the TARDIS to Castrovalva while Nyssa returns the Doctor to the Zero Room. The Doctor, sceptical that Tegan can actually pilot the complex time machine, is convinced that someone else is responsible for the journey - although he can't think of who might be. But he soon has a greater problem, as Nyssa opens the doors of the Zero Room to reveal that it has been jettisoned.

Tegan apparently pilots the TARDIS to Castrovalva and lands in the forest outside, materialising the ship on an angle. Meanwhile, the Doctor has given his sonic screwdriver to Nyssa and told her to take the doors off the wall - and when the ship touches down the door of the Zero Room falls off its loosened hinges and balances against the wall. The Doctor, partially shielded underneath, begins to recover and is finally able to explain that they can build a Zero Cabinet out of the pieces of the Zero Room which remain. Nyssa and Tegan then prepare to carry the Zero Cabinet to Castrovalva, balancing it on the electric wheelchair; but as the journey progresses they accidentally knock the wheelchair into a creek, shorting out its motor and locking its wheels. Nyssa falls into the creek while retrieving the wheelchair and finds that her ion bonder is now waterlogged. Unable to repair the wheelchair, they are forced to continue their journey on foot - unaware that they are being observed.

As evening approaches, Tegan spots Castrovalva in the distance, a walled city atop a tall cliff. Nyssa hides the Zero Cabinet under a covering of leaves, just in case, and she and Tegan set off to look for a way in - unaware that nearby, spear-carrying warriors dressed in feathered suits of armour have observed their arrival and are discussing what to do about it. The Doctor, unaware of the warriors' presence, opens the Cabinet. Soon afterwards, Nyssa and Tegan return, having failed to find an easy way into the city - only to find blood on the ground, the Cabinet open, and the Doctor missing...

Part three
Nyssa and Tegan follow the trail of blood away from the Cabinet, trying to find the Doctor while avoiding the feathered warriors in the forest. They eventually spot the Doctor some distance away, but although they call out to him he doesn't recognise his own name -- all he knows is that he, too, is looking for someone called "the Doctor". He follows the trail of blood to a doorway in the cliff, where a group of warriors who have killed a wild pig are waiting for their fellow, Ruther, to arrive. Ruther's group arrives carrying the Zero Cabinet, and Ruther finds the Doctor hiding in the bushes. The warriors take the Doctor into Castrovalva with them, and Nyssa and Tegan arrive seconds too late as the door seals itself shut.

The Doctor is surprised to find that Castrovalva is a pleasant, villa-like city with an open courtyard, tiled floors, and soft pastel colours in its walls and pillars. Furthermore, the "warriors" are all, in fact, placid, middle-aged men who are greeted upon their return by Shardovan, the dour town librarian. Castrovalva is in truth a most civilised town, and its inhabitants were merely engaged in an exercise programme and were wearing old suits of armour to stiffen their resolve for the hunt; Shardovan did not go as he is too tall to fit into the available armour. The Doctor, still uncertain as to his own identity, is taken to the guest rooms to spend the night; there, he is greeted by the Portreeve, a kind elderly man who oversees the daily life of Castrovalva. The Doctor drinks a medicinal tonic prescribed by the town's herbalist, Mergrave, and goes to sleep, assured by the Portreeve that he'll soon find the Doctor he seeks.

Nyssa and Tegan scale the walls to get into Castrovalva and are surprised when a rope ladder is lowered down to them. Inside, Shardovan decides not to tell the Portreeve about the new arrivals and is unpleasantly surprised to find that the Portreeve is already aware of them and has been standing on the walkways above the courtyard all this time. Mergrave shows Tegan and Nyssa that the Doctor is sleeping peacefully and shows them to their own guest quarters. But Adric emerges from the shadows in the Doctor's room and watches them go...

The next morning, Nyssa leaves Tegan sleeping peacefully and sets off to explore Castrovalva. While passing through the courtyard she encounters a group of Castrovalvans carrying the Zero Cabinet and redirects them to the Doctor's room. There, Adric appears to her, and warns her not to tell the Doctor about his kidnapping; it's important that the Doctor remains in Castrovalva until his regeneration is complete. Adric's image then breaks up, as the Doctor awakens, feeling much better. The Master is satisfied; now, they will remain untroubled by the Doctor's meddling.

The Doctor, Tegan and Nyssa have breakfast with the Portreeve, and then Nyssa and Tegan set off to search the town library for books on telebiogenesis. Unfortunately, the library has no technical section, and they decide to read up on the history of Castrovalva instead. Meanwhile, the Portreeve shows the Doctor the source of his knowledge: an ancient tapestry designed with long-forgotten techniques, which reweaves itself to display events of great importance such as the Doctor's arrival. As the Doctor watches the images of Tegan and Nyssa carrying him through the forest he begins to get the impression that someone's missing, and a chance encounter with a young girl inspires him to remember Adric.

The Doctor returns to his room to confront Tegan and Nyssa, who are forced to admit what has happened. The Doctor insists upon leaving Castrovalva immediately to rescue Adric. They pass through the town square, where the women of Castrovalva are doing their laundry - but find their path leading impossibly back to the square, no matter which direction they take. The Doctor collapses, suffering a relapse as a complex spatial distortion begins to take effect around him. Tegan and Nyssa manage to get him back to his room, only to find that the Zero Cabinet has been removed - and outside the window, the distortion of Castrovalva is much more apparent. The town is deliberately folding in on itself, and the Doctor and his companions are trapped...

Part four
The Doctor collapses, urging Nyssa and Tegan to find the Zero Cabinet. For the moment, he is able to block out the effect of the spatial anomaly by placing a silver-backed mirror in front of the window. While waiting for his companions to return, he reads through the history of Castrovalva and finds a piece of paper with some very curious implications. Mergrave arrives, and the Doctor questions him about the geography of Castrovalva. He seems not to understand the Doctor's confusion until the Doctor has him draw a map of Castrovalva on the back of the mirror — and he is taken aback to discover that he can locate his single pharmacy in four different locations on the map.

Ruther leads Nyssa and Tegan in circles about the town but seems to see nothing amiss. Tegan is convinced that the people of Castrovalva are all involved in a conspiracy, but Nyssa suspects that they are also a part of the mathematical recursion affecting the town. They finally realise that the women in the square are doing their laundry in the Zero Cabinet and demand that it be returned to the Doctor. There, the Doctor tests Ruther, who is also surprised to find that he can find a single location in four places on the map. The Doctor is convinced that he is overlooking something obvious about the history of Castrovalva - and the notes he has found, in Shardovan's handwriting, indicate that Shardovan also knows what it is. The Doctor suspects that the books are forgeries, to cover up something about Castrovalva's real history — if indeed there is one.

The Doctor requires answers from the Portreeve but must be carried to his home in the Zero Cabinet; but while Tegan ushers Mergrave and Ruther out of the room, the Doctor suggests an alternate plan to Nyssa. When Tegan and Nyssa emerge from the Doctor's room with the Cabinet, Tegan rebuffs Shardovan's offer to help carry the Cabinet. Shardovan is then lured away from the procession and confronted by the Doctor, who filled the Cabinet with all thirty volumes of the history of Castrovalva. He knows from Shardovan's notes and refusal to join the hunt that Shardovan suspects the whole history of Castrovalva is a fiction; but even the Doctor fails to realise the extent of the fiction until Shardovan points out the obvious fact he has overlooked: the books are five hundred years old, but they chronicle the history of Castrovalva to the present day! Shardovan takes the Doctor to the Portreeve's home, to break in and confirm the Doctor's suspicions about the true nature of Castrovalva.

The Portreeve dismisses all townspeople but Mergrave and Ruther, who stand by in confusion as the Portreeve removes his disguise, revealing himself to be the Master. Castrovalva is, in reality, a Block Transfer Computation, brought into existence and maintained by Adric's calculations; the entry in the TARDIS data bank was planted by Adric to lure the Doctor into the trap. The Master attempts to break into the Zero Cabinet, insanely determined to have one last look at the Doctor's face before he destroys him, but the Doctor breaks in and confronts him. The Doctor rips down the ever-changing tapestry to reveal Adric pinned behind it in a web of power lines. The Doctor is powerless to stop the Master as he begins to dispose of the bait, casually causing Ruther to vanish when Ruther tries to attack him. But Shardovan demonstrates his free will by swinging from the balcony on the chandelier into the web, giving his life to break the strands and release Adric.

Without the support of Adric's constant calculation, the computation that is Castrovalva begins to collapse in upon itself. The Master tries to flee in his TARDIS, which was disguised as the Portreeve's fireplace, but space is folding in too fast and his TARDIS is unable to escape from Castrovalva. Mergrave tries to lead the Doctor and his friends to safety but soon even he can no longer find his way about — but Adric, who created Castrovalva, can see clearly and guides his friends out through a breach in space onto the hillside. As the Master rushes for the breach, Mergrave remains to hold him back, and the Doctor is forced to watch helplessly as the breach closes up, trapping the Master along with the doomed Castrovalvans.

As Castrovalva disappears permanently, the Doctor and his companions jog back to the TARDIS. There, the Doctor reveals to Tegan's disappointment that she did not pilot the TARDIS after all; the whole flight had been pre-programmed by Adric and the Master; of the latter, the Doctor says, "The Master leaves nothing to chance." But she need not worry about piloting the TARDIS away; the Doctor's regeneration has finally concluded, and he cheerfully says that "Whoever I feel like, it's absolutely splendid."

Cast

 * The Doctor - Peter Davison
 * Adric - Matthew Waterhouse
 * Nyssa - Sarah Sutton
 * Tegan - Janet Fielding
 * The Master - Anthony Ainley
 * Shardovan - Derek Waring
 * Mergrave - Michael Sheard
 * Ruther - Frank Wylie
 * The Portreeve - "Neil Toynay"
 * Head of Security - Dallas Cavell
 * Child - Souska John

Uncredited cast

 * The Doctor - Tom Baker
 * Security Guards - Kenneth Lawrie, Harry Fielder (DWM 258)
 * Ambulance Men - Peter Roy, Derek Matt (DWM 258)
 * Castrovalvan Warriors - Cy Town, Mark Allington, Ernie Goodyear, Colin Cook, Dave Harrison, Ray Martin (DWM 258)
 * Stunt Warrior - Stuart Fell (DWM 258)

Crew

 * Assistant Floor Manager - Renny Tasker
 * Costumes - Odile Dicks-Mireaux
 * Designer - Janet Budden
 * Film Cameraman - John Baker
 * Film Editor - Mike Houghton, Robin Jackman
 * Film Sound - Jim McAlister
 * Incidental Music - Paddy Kingsland
 * Make-Up - Marion Richards
 * Production Assistant - Olivia Cripps
 * Production Manager - Margot Hayhoe
 * Production Associate - Angela Smith
 * Script Editor - Eric Saward
 * Senior Cameraman - Alec Wheal
 * Special Sounds - Dick Mills
 * Studio Lighting - Ron Bristow
 * Studio Sound - Laurie Taylor
 * Technical Manager - Clive Gulliver
 * Theme Arrangement - Peter Howell
 * Title Music - Ron Grainer
 * Videotape Editor - Rod Waldron
 * Video Effects - Dave Chapman
 * Vision Mixer - Carol Johnson
 * Visual Effects - Simon McDonald
 * Writer - Christopher H. Bidmead
 * Producer - John Nathan-Turner
 * Director - Fiona Cumming

Uncredited crew

 * Graphics - Ian Hewitt (INFO: Castrovalva)
 * Booking Assistant - Sheila Hodges (INFO: Castrovalva)
 * Floor Assistant - Sarah Woodside (INFO: Castrovalva)
 * Film Camera Assistant - Paul Roy (INFO: Castrovalva)
 * Visual Effects Assistants - Malcolm James, Ron Thornton, George Reed (INFO: Castrovalva)
 * Studio Engineer - Dan Shaw (INFO: Castrovalva)
 * Show Working Supervisor - Don Evans (INFO: Castrovalva)
 * Lighting Chargehand - John Morin (INFO: Castrovalva)
 * Dressers - Alison Cowles, Liz Dixon, Ray Greenhill, Tim Williamson (INFO: Castrovalva)
 * Props Buyer - Helen MacKenzie (INFO: Castrovalva)
 * Film Operations Manager - Graham Richmond (INFO: Castrovalva)
 * Film Ops - John McEroy, Dick Norwood, Dicky Rickes, John Shayer, Brian Simmons, Brian Walter (INFO: Castrovalva)
 * Grips - Jimmy Monks (INFO: Castrovalva)
 * Film Sound Assistant - Gordon Lester (INFO: Castrovalva)
 * Film Lighting - Ted Turpin (INFO: Castrovalva)
 * Grams Operator - John Downes (INFO: Castrovalva)
 * Make-Up Assistants - Lesley Bond, Sula Loizou, Marilyn MacDonald, Charlotte Norman, Lucy Wiles (INFO: Castrovalva)
 * Design Assistant - Rod McLean (INFO: Castrovalva)
 * Production Secretaries - Fiona Duncan, Jane Judge (INFO: Castrovalva)

Food and beverages

 * Mergrave offers a tonic drink to the Doctor, who recognises Valeriana officinalis, Sanicula europaea, and a hint of rosemary among its ingredients.

The Doctor

 * The Doctor has recently regenerated from his fourth incarnation.
 * While he is still disorientated, the Doctor addresses Adric as "Brigadier" and "Jamie", and Tegan as "Vicki" and "Jo". He mentions the Ice Warriors and K9 as if they were present. He urges Tegan and Nyssa not to "reverse the polarity of the neutron flow" — a catchphrase traditionally associated with the Third Doctor — and toys with a flute-recorder, a trademark of the Second Doctor, whose persona he also briefly adopts. He mentions visiting Alzarius and Logopolis. He adopts the persona of the First Doctor when wandering through the TARDIS, grasping his lapels and referring to Adric as "boy", and has the characteristic laugh of the First Doctor. He also mentions that Romana keeps telling him "to take a holiday" and speaks as if she is still there; it is only when Adric reminds him that she is gone that he remembers.
 * Also in his delirium he says, "Oh, the Ogrons and the Daleks. I think it does us good to remind ourselves that the universe isn't entirely peopled with nasty creatures out for themselves".
 * The Doctor levitates himself to make the Zero Cabinet easier for Tegan and Nyssa to carry.
 * The Doctor is able to tell the age of a volume of the chronicles of Castrovalva by smelling it. He is also able to determine the individual contents of Mergrave's remedy by tasting it.

The Doctor's items

 * The Doctor provides chalk to draw a map of Castrovalva.
 * The Doctor finds a recorder in the TARDIS before finding a cricket bat.

Gallifrey

 * The Doctor says that there is a polygonal Zero Room under the Junior Senate block on Gallifrey.

Individuals

 * Nyssa confesses she does not know enough telebiogenesis.

Power sources

 * The Master uses a Hadron web, which is powered by Adric, to power his creation, Castrovalva.

TARDIS

 * The Zero Room is "an isolated place cut off from the rest of the universe" which is used by Time Lords after difficult regenerations.
 * As the TARDIS travels back to the beginning of the universe the Cloister bell sounds throughout the TARDIS.
 * The Master uses his TARDIS to stun Adric and the guards from the Pharos Project.
 * The Masters' TARDIS chameleon circuit is originally disguised as a column and then later as a fireplace.
 * As the TARDIS approaches Event One, it is nearly destroyed.

TARDIS components

 * Architectural Configuration allows the Doctor, Nyssa and Tegan to delete parts of the TARDIS to escape Event One.
 * The TARDIS information system (after the Master's tampering) uses two letter inputs; e.g. IF for Index File.
 * The Thermo-buffer is a part of the TARDIS that needed to be vented should the ship overheat.

Theories and concepts

 * Logopolitans utilise Block Transfer Computations.
 * Tegan says to herself and Nyssa, "If we had the Index File, we could look it up in the Index File, under 'Index File'." Nyssa says that turn of thought is an example of recursion.
 * Event One, the creation of the universe (mistakenly called "the galaxy" in the Index File and by Nyssa) is equated with a hydrogen in-rush.

Story notes

 * Christopher H. Bidmead's outline was entitled The Visitor, but his full scripts weren't. After he actually sold his pitch, the story was commissioned and thereafter always known as Castrovalva. (REF: The Fifth Doctor Handbook)
 * This is the first story to have the Doctor credited as "The Doctor" (rather than previously as "Doctor Who" or simply "Dr. Who"). With the exception of series 1 of the 2005 revival, this practice would continue hereafter.
 * For this story, the series was shifted from its traditional Saturday early evening transmission to a twice-weekly (Monday and Tuesday) slot. However, the format change was not well promoted, with the result that many regular viewers missed part two. According to production notes on the DVD release, some parts of the UK saw part one at a different time than the scheduled BBC broadcast, in some cases as early as mid-afternoon.
 * The Portreeve is listed in the credits as being played by "Neil Toynay", an anagram of "Tony Ainley". This was a play on the Master's habit of using either anagrams or synonyms for "Master" as aliases, used here to make the reveal of the Porteeve's identity a surprise, and the trick would later be used in Time-Flight, albeit under a different pseudonym. Fiona Cumming's husband came up with the "Neil Toynay" alias.
 * The shot used in this story of the town of Castrovalva sitting on a cliffside, and the name of the settlement, was inspired by a print by M. C. Escher. Later in the serial, the collapsing world of Castrovalva takes on a distorted appearance, reminiscent of Escher's surrealist work.
 * For the final scene, the script called for Adric to look "pallid" as he was still recovering from the effects of imprisonment by the Master. According to the commentary on the DVD, this was accidentally achieved by Matthew Waterhouse, who had a hangover from the night before from drinking too much Campari. Whilst the cameras were filming the Doctor and Tegan in conversation about who landed the TARDIS, Waterhouse was being sick behind a tree.
 * When filming a scene that required the Doctor to eat a piece of celery, Peter Davison immediately spat it out on the first take, due to his intense dislike of the vegetable.
 * Part one was the first-ever Doctor Who episode to include a pre-credits sequence.
 * The Radio Times programme listing for part one was accompanied by a black-and-white head-and-shoulders shot — labelled BBC tv NEW SEASON — of the new Doctor examining the unravelled scarf of his previous incarnation, with the accompanying caption " 'The new Doctor's coming unravelled in more ways than one', and his regeneration isn't working too well either! Doctor Who returns at 6.55".
 * During part two, Nyssa's costume gradually transforms into the standard outfit she would wear for the rest of the 1982 season: first, she exchanges her "fairy skirt" for a pair of trousers when she falls into a brook while attempting to catch the runaway wheelchair before she and Tegan begin carrying the Zero Cabinet to Castrovalva; along the way, she abandons her velvet jacket, and finally she loses her ornamental hair-comb when it gets caught by a tree branch. By the time she and Tegan reach the base of the cliff, she is more-or-less outfitted as she would be for the remainder of the season. This was mainly done behind the scenes to make her outfit more practical for regular filming.
 * The decision to have the Doctor impersonate his previous incarnations was made during rehearsal when Peter Davison presented impersonations as part of his preparation for taking on the role; his interpretation was intended to combine elements of the past Doctors.
 * Scotland was the first country to see the Fifth Doctor; BBC1 Scotland aired the first episode on 4 January 1982 at 3.30pm. (REF: )
 * This is the first Fifth Doctor story aired, although it was not the first to be filmed - TV: Four to Doomsday, the following story, was first. Castrovalva was in fact filmed fourth.
 * Tom Baker appears in the opening sequence, a redo of the regeneration scene which ended TV: Logopolis, but is uncredited both on-screen and in Radio Times.
 * Strangely, unlike most recaps of a cliffhanger at the time, this one was a reshoot of the final minutes of Logopolis, similarly to the recaps of the first six seasons, rather than a simple replay (though the footage of Baker was a replay). As a result, numerous differences (documented in "Production Errors") are present.
 * New opening and closing title sequences are used, similar to the previous season's and again designed by Sid Sutton but this time incorporating Peter Davison's face in place of Tom Baker's.
 * The fact that the Fifth Doctor is not wearing the same shoes as those his predecessor wore is not without precedent in the history of regeneration stories. The Second Doctor appeared to have "regenerated" items of clothing along with his body when he emerged in The Power of the Daleks. There is no scene in that story where the Second Doctor changes clothes; the First Doctor's clothes have ostensibly regenerated with him. Likewise, the Fourth Doctor has different shoes and a different jacket from the Third's Spearhead from Space kit. The difference in Castrovalva is that the production team admitted to simply making a continuity error; they were not deliberately trying to imply — as arguably Innes Lloyd's Power of the Daleks team were — that the clothes regenerated with the man. (DCOM: Castrovalva)
 * The story was the first of only four Doctor Who serials in the 1963-89 series to feature a pre-credits sequence (the others were TV: The Five Doctors, Time and the Rani and Remembrance of the Daleks). In the 2005 series, pre-title sequences became a regular feature.
 * The reprise from the final moments of the preceding story, Logopolis, had the incidental music changed from its original sombre melody, signifying the 'death' of the Fourth Doctor, to a more upbeat sound, signifying the 'birth' of the Fifth.
 * From this story until its final regular appearance (as well as a one off return), the red knobbed lever is established as the main door control. Previously it was either a generic lever or served a different function (such as the randomiser in The Armageddon Factor or taking the TARDIS out of time/space in Logopolis) and usually in a different position on the console.
 * This is the only post-regeneration story to be directed by a woman.
 * This was the only season opener in the Fifth Doctor era that wasn't written by Johnny Byrne.
 * In crafting his storyline, Christopher H. Bidmead remembered two prints in the office of Graeme MacDonald, the Head of Drama, whose optically illusory nature had irritated John Nathan-Turner who believed that "art should be there to soothe, not distract". These had been drawn by M. C. Escher, in which figures are seen to walk up or down all sides of a series of staircases, Belvedere (1958), in which the perspective of a building changes between floors, and Ascending and Descending (1960), in which a staircase endlessly loops back onto itself. Bidmead thought that an environment exhibiting these sorts of traits could complement a story which dealt with the mathematical concept of recursion, in which one member of a sequence is generated by one or more of the preceding members of that sequence.
 * Peter Davison requested that his character start being credited as "The Doctor" from Season 19 onwards, rather than the traditional "Dr. Who"/"Doctor Who", as the older credit was incorrect in the context of the show's continuity. This change would be upheld all the way until the 1996 TV movie, before switching back to the credit of "Doctor Who" in the revival series. This would again be changed to "The Doctor" once David Tennant took on the role, at his request due to "Doctor Who" being, as he deeemed, incorrect.
 * In the screenplay for part two, Tegan was scripted to take a drink fresh from the stream they were near while Nyssa attempted to repair the wheelchair — a scene dropped from the shooting schedule over fears that younger children might imitate this. In the 2018 "Making of" documentary for the episode's Blu-Ray release, Janet Fielding revealed that she refused to shoot that scene as scripted at all; her father was a parasitologist, and she knew full well what could be lurking in river water.
 * Elisabeth Sladen and Louise Jameson were each asked to reprise their roles as Sarah Jane Smith and Leela respectively in order to ease an audience into a new Doctor. However, both of them turned down the offer due to scheduling commitments to other projects.
 * Peter Davison liked the story's complexity, but wasn't sure what it was about.
 * Fiona Cumming had to shoot the cliff scenes carefully due to Janet Fielding's severe fear of heights, framing the action to make it appear as if the actors were at a much greater elevation than was really the case.
 * Patrick Troughton visited the set and stood in for Peter Davison during camera rehearsals while the star was filming Sink or Swim.
 * Souska John was Caroline John's niece.
 * An unintended consequence of making three other serials prior to this one was that Fiona Cumming had to encourage the series regulars to behave with less familiarity towards one another.
 * Upon arriving at Buckhurst Park, John Nathan-Turner greeted a man he assumed to be the gardener, only to discover that he was actually addressing the estate's owner, William Herbrand Sackville, the tenth Earl De La Warr. Despite this case of mistaken identity, Earl De La Warr was delighted to have Doctor Who filming on his premises; in lieu of receiving a fee for the use of his property, he had agreed to a charity donation and a photo in front of the TARDIS.

Myths

 * Castrovalva was recorded fourth in season 19 in order to give Peter Davison more time to grow accustomed to playing the Doctor before filming his first adventure.
 * Though Peter Davison himself is in part responsible for this myth (DCOM: Four to Doomsday, Castrovalva and others), there's little truth in it. In about late February 1981, JN-T cancelled what should have been Davison's first script, Project Zeta-Sigma. This threw the production office a significant curveball, as Davison was due to begin recording in mid-April. On 9 March, Christopher H. Bidmead was commissioned to write an outline for the emergency replacement story. It wasn't until 8 April that the full scripts were commissioned. In other words, Castrovalva wasn't even written at the time Davison first donned his celery. (REF: The Fifth Doctor Handbook)

Ratings

 * Part one - 10.1 million viewers
 * Part two - 8.7 million viewers
 * Part three - 10.4 million viewers
 * Part four - 10.5 million viewers

Filming locations

 * Crowborough Wireless Telegraph Station (now known as Sussex Police Training Centre (Crowborough)), Duddleswell, East Sussex
 * Harrison's Rocks, Groombridge, East Sussex
 * Estate of Lord De La Warr, Buckhurst Park at Withyam, East Sussex
 * BBC Television Centre (TC1 &amp; TC6), Shepherd's Bush, London

Between Logopolis and Castrovalva
The first episode of this story is narratively continuous with the last episode of Logopolis.
 * Peter Davison's Fourth Doctor costume has different shoes than Tom Baker's; Baker was wearing brown leather boots when he regenerated; Davison was wearing ankle-high shoes thereafter. This was pointed out by Davison on the DVD commentary.
 * The TARDIS is in a different field from its previously seen position in Logopolis (though it is not without precedent for the TARDIS to relocate autonomously).
 * Director Fiona Cumming cast different actors for the security guards chasing the companions at the start of episode one than Peter Grimwade had used at the end of Logopolis.
 * Nyssa and Tegan's handbags appear in the Control room, not having been there in their final interior TARDIS scenes in Logopolis.
 * Peter Davison's hair is light brown at the end of Logopolis and the beginning of Castrovalva, when he first appears as the newly regenerated Doctor, but has changed to near platinum blond as he is helped through the field to the TARDIS at the start of Castrovalva (though this could be explained as being a post-regenerative side effect). In actuality, this was done to make his hair contrast with Tom Baker's hair.
 * Tegan's hairstyle is shorter, curlier, and darker in the Logopolis footage, becoming noticeably longer, straighter, and more auburn at the beginning of Castrovalva.

Problems unique to Castrovalva

 * The ambulance that arrives for the Doctor is from the East Sussex Health Authority, despite the Pharos Project supposedly being located in Cambridgeshire.
 * In the Zero Room sequence, the image is horizontally flipped. This has the effect of making the Doctor's collar question marks appear reversed. The error was gleefully pointed out by Peter Davison in the DVD commentary.
 * When Tegan lands the TARDIS, she ends up falling to the right of the console room, and yet Nyssa fell to the left when they landed even though the Zero Room is pretty much straight down from the console room.
 * On Castrovalva, the camera wobbles when Mergrave and Ruther go to see what caused a noise.

Continuity

 * The Fifth Doctor first dons his cricket uniform and his habit of pinning a stalk of celery to his lapel, although he doesn't explain why until TV: The Caves of Androzani.
 * Immediately after leaving Castrovalva, the Doctor, Adric, Nyssa and Tegan travelled to the Psychodrome. (AUDIO: Psychodrome)
 * The TARDIS outruns Event One, implied to be the Big Bang. Ironically, it would end up at the epicentre of Big Bang Two, being its cause. (TV: The Big Bang)
 * The Zero Room was later reconstructed in the TARDIS. (AUDIO: Zaltys, Renaissance of the Daleks) By the time of the Doctor's eighth incarnation, however, the TARDIS once again lacked a Zero Room. (AUDIO: Day of the Vashta Nerada)
 * The Doctor is capable of levitating himself. (COMIC: Levitation)
 * When imitating his second incarnation whilst in a delirious state, the Doctor references an adventure with the Ice Warriors and the Brigadier (AUDIO: Wrath of the Ice Warriors)

Blu-ray releases
This story was released on Blu-ray as part of The Collection: Season 19 Blu-ray box set. Released:
 * UK Release - 10 December 2018
 * US Release - 4 December 2018
 * AUS Release - 23 January 2019

DVD releases
This story was released on DVD together with The Keeper of Traken and Logopolis as part of the New Beginnings DVD box set. Released:
 * Region 2 - 29 January 2007
 * Region 4 - 7 March 2007
 * Region 1 - 5 June 2007

Contents:
 * Audio Commentary by actors Peter Davison and Janet Fielding, writer Christopher H. Bidmead and director Fiona Cumming
 * Being Doctor Who - Peter Davison discusses how he approached this iconic role.
 *  Directing Castrovalva - Fiona Cumming talks about her work directing Peter Davison's debut story.
 * The Crowded TARDIS - A look at the increase in the TARDIS crew, with Tom Baker, Peter Davison, Sarah Sutton, director John Black and Christopher H. Bidmead.
 * Swap Shop & Blue Peter - Peter Davison is interviewed on these two long-running magazine programmes.
 * Deleted Scenes - Two deleted sequences from the location filming.
 * Theme Music Video - A brand new remix is Peter Howell's Doctor Who theme music in stereo or Dolby 5.1 surround.
 * Trailers & Continuity Announcements
 * DVD-ROM feature
 * Radio Times Billings - Articles and listings from Radio Times
 * Doctor Who Annual 1982
 * BBC Enterprises literature
 * Photo Gallery
 * Easter Egg - On the main menu, navigate to Audio Options and press left to highlight the Doctor Who logo. Press Enter/OK to be treated to a short clip of the Fifth Doctor on a float with assorted monsters at the Lord Mayor of London's Parade on 1 January 1982.
 * Production subtitles

Notes:
 * Editing for the DVD release was completed by the Doctor Who Restoration Team.

Doctor Who DVD Files
It was released with issue 47 of Doctor Who DVD Files.

Video releases
This story was released on video in 1992 in the UK and Australia and 1993 in the US.

Digital releases

 * The story is available for streaming in Canada & the US through Britbox.
 * It is also available to download through iTunes.