Istanbul

Istanbul, also known at various points as Constantinople and Byzantium, was a city on Earth which was located on both the continents of Asia and Europe. (PROSE: Byzantium!)

Most travellers, at least in the 1st century, would access the city by passing from the Aegean, through the Hellespont and across the Sea of Marmara, where the city lay. (PROSE: Byzantium!)

Geography
The west side of the city was the tip of Europe. In the 1st century Byzantium, this was the location of the Greek and Jewish quarters, as well as the Hellenic-Roman city centre. Located there were the Theodosian forum, temples, pavilions, public baths and amphitheatres. The walls of this side of the city were incredibly strong, and met the sea at the Porta Aurea. (PROSE: Byzantium!)

Byzantium
Byzantium was, according to Bernice Summerfield, the oldest name of Istanbul. (PROSE: Shadowmind) The earliest settlers in the region were Dorian Greeks who sheltered their flocks there in the winter. Later, the Spartans took the city, fortifying the it into a stronghold. Like many other Greek cities, it was a city-state at first. It was usually democratic, although at times it was ruled by oligarchies and tyrants. During this time, Byzantium withstood separate attacks from the Gauls and the Seleucid king Antiochus I.

Eventually the Romans arrived in the city during the 2nd century BC, with Byzantium falling in a bloodless surrender. The two then entered into an alliance, where Byzantium retained its status as a free-city and gained the protection of Rome, albeit while paying an annual tribute. A Roman garrison was placed on the outskirts of the city.

During the reign of Emperor Augustus, Byzantium was striped of its possessions in Bithynia, with the Roman garrison moved to the town itself. While the city technically remained a city-state, the Romans were very much in ruling the city by this point.

The First Doctor, Barbara, Vicki and Ian visited the city in 64 AD, and found it to be a city struggling with three groups vying for power: the Jewish Pharisees, the Roman Empire and the adherents of the fledgling Christian religion. (PROSE: Byzantium!)

Byzantium begat New Byzantium. The Seventh Doctor once told Ace that he thought New Byzantium stole the name because the two cities had a similar geography, both cities being set "on an isthmus of land, cut through by a strait linking two seas". (PROSE: Shadowmind)

Constantinople
The name switched to Constantinople in the early part of the first millennium. Benny once told Ace that the name switch was in honour of Emperor Constantine. She went on to say that Constantinople was "the capital of the Eastern Roman Empire until the 15th century, when it was captured by the Turks". (PROSE: Shadowmind)

In the 3rd century, an incarnation of the Doctor encountered Fenric in Constantinople, where he defeated him at chess and trapped him in a flask. (TV: The Curse of Fenric) By his fourth incarnation, the Doctor bought a hookah in Constantinople, which he would later use as part of a ruse to evade the Chancellery Guard on Gallifrey. (TV: The Deadly Assassin)

In 540, the First Doctor, Vicki and Steven Taylor visited Constantinople. However, through the machinations of the Monk to sabotage the Doctor's timeline, the First Doctor's place was taken by the Fifth Doctor. This damage would later be repaired. (AUDIO: The Secret History)

In the 10th century, the Fifth Doctor and Peri Brown visited Constantinople. (PROSE: Methuselah)

Piloting the Doctor's TARDIS, Nardole visited 12th century Constantinople and ruled for a while as Emperor, as one of "a few accident stop-offs" before successfully retrieving the Twelfth Doctor from Harmony Shoal's head office in 21st century New York. (TV: The Return of Doctor Mysterio)

In 1914, the Seventh Doctor and Ace visited Constantinople to locate and dispose of a malfunctioning Krivani interstellar drive. (PROSE: Ante Bellum)

In 1924, River Song met Bertie Potts while their boat, headed to an archaeological dig in Mesopotamia, was docked in the city. (AUDIO: The Boundless Sea)

At some point, Constantinople had a cash and carry that the Fourth Doctor believed to be "far superior" to P. Fords in New York City. (COMIC: Time Lady of Means) Prior to his visit to Quinnis, the First Doctor acquired Turkish delights from Constantinople. (AUDIO: Quinnis)

Istanbul
By 1963, Constantinople had become Istanbul. Susan Foreman, while taking school on Earth, once forgot the time and called the city Constantinople. In consequence, her geography teacher made his entire 4G class chant, "It's Istanbul, not Constantinople!" (PROSE: Time and Relative)

In the 21st century, the Ninth Doctor fought Sontarans on a rooftop in Istanbul. Sally Sparrow saved his life from one of them, and gave him her Christmas homework for instructions for when the TARDIS would "burp" and jump forward in time twenty years without the Doctor. (PROSE: What I Did on My Christmas Holidays by Sally Sparrow)

Byzantium again
The city was again known as Byzantium by the time of the Earth Alliance's war with the Eminence. The Alliance's media centre was located there. (AUDIO: Rule of the Eminence)

Behind the scenes

 * Narratives in the Doctor Who universe have not, as of 2019, particularly defined when the city's name changed.
 * According to the pseudo-narrative Torchwood website, 4th century BC Istanbul had sewers full of Weevils.