A city an emperor renamed for himself, a sultan conquered for his, and neither name outlasted the people who kept crossing between two continents anyway.

Greek colonists from Megara are traditionally said to have founded Byzantium on the site around 657 BC, drawn by a natural harbour on the Golden Horn commanding the strait between the Black Sea and the Mediterranean. The Roman Emperor Constantine the Great rebuilt the city on a vastly greater scale and dedicated it in 330 AD as his new capital, Constantinopolis — “Constantine’s city” — a decision that split the Roman world’s centre of gravity eastward and, after the western empire’s collapse in 476, left Constantinople as the sole capital of what modern historians call the Byzantine Empire for a further millennium. The Emperor Justinian rebuilt the Hagia Sophia as the world’s largest cathedral in 537 after riots had burned its predecessor, and the city’s fortified walls, built under Theodosius II in the 5th century, held off repeated sieges for eight hundred years, broken only once, catastrophically, when the Fourth Crusade sacked the city in 1204 and installed a short-lived Latin Empire before a restored Byzantine dynasty reclaimed it in 1261.

A weakened Byzantium finally fell on 29 May 1453, when the twenty-one-year-old Ottoman Sultan Mehmed II, thereafter known as Fatih — “the Conqueror” — took the city after a fifty-three-day siege, ending the Byzantine Empire and installing Constantinople as capital of the Ottoman Empire in its place; the Hagia Sophia was converted to a mosque within days, and Mehmed set about repopulating and rebuilding a city depleted by decades of decline. The Ottomans ruled from Constantinople for the better part of five centuries, adding Topkapi Palace as the sultans’ seat and, in the 19th century, the French-and-Ottoman-styled Dolmabahçe Palace on the Bosphorus shore, before the empire’s defeat in the First World War brought Allied occupation and, in 1923, Mustafa Kemal Atatürk’s new Republic of Turkey, which moved the capital to Ankara rather than the old imperial seat. The city formally adopted the name Istanbul in a 1930 postal reform, the Hagia Sophia was secularised into a museum in 1935 before returning to use as a mosque in 2020, and the strait the city straddles still carries its old function forward: a working crossing between Europe and Asia, no longer the capital of an empire, but still the only city in the world built across two continents.