Middle Ages

The Middle Ages were an era in human history following the Dark Ages. The Sixth Doctor believed that it was "an interesting period in its way but in increasingly small doses." (AUDIO: Leviathan)

The First Doctor, Susan, Barbara, and Ian's trip to Cathay in 1289 (TV: Marco Polo was later referred to by Ian as being in the Middle Ages. (PROSE: The Sorcerer's Apprentice)

Stangmoor Prison was a fortress in the Middle Ages. (TV: The Mind of Evil)

Sometime in this era, the Fifth Doctor and Vislor Turlough visited the village of Vadhoc and witnessed the storytelling of Grayling Frimlish. (AUDIO: The Last Fairy Tale)

Broton, while disguised as the Duke of Forgill, claimed that there had been reported sightings of the Loch Ness Monster since the Middle Ages. (TV: Terror of the Zygons)

Leeds Castle was built in the Middle Ages. (PROSE: Legacy of the Daleks)

At some point during the Middle Ages, the planet Strepto vanished. (PROSE: Something in the Water)

Compared to the 21st century, Britain was on average three degrees colder in the Middle Ages. (TV: The Last Oak Tree)

Upon seeing a Sontaran teleport pod in the Rattigan Academy in 2009, the Tenth Doctor told Luke Rattigan that "it might be Earth technology, but that's like finding a mobile phone in the Middle Ages," pointing out that the presence of such futuristic technology was an anachronism. (TV: The Sontaran Stratagem)

Also in 2009, Elijah Spellman told Sarah Jane Smith that mankind had always needed someone to make them laugh, slave or king. In Great Britain they had fools like the harlequin and, in the Middle Ages the jester. (PROSE: Day of the Clown)

Steven Taylor believed that Leonardo da Vinci lived in the Middle Ages. (TV: The Time Meddler)

Behind the scenes
The non-narrative source Doctor Who: The Time Traveller's Almanac states that the Daleks had stolen Strepto, making it one of the twenty-seven planets they needed to activate the reality bomb. This is contradicted by The Stolen Earth, which counts twenty-four planets being taken from 2009 and the other three from other eras named by the Tenth Doctor and Donna as the Lost Moon of Poosh, Pyrovillia and Adipose 3.