Constantinople (Byzantine Greek: Konstantinoupolis; Latin: Constantinopolis or Byzantium; Turkish: Kostantiniyye or İstanbul) was the capital of the Byzantine Empire and, following the city's conquest in 1453, became the Ottoman Empire's capital in 1458.
In 1204, the city was visited by the Mentor of the Levantine Assassins, Altaïr Ibn-La'Ahad, who had hopes of introducing the Assassin Brotherhood to the city. However, civil disobedience and mass riots, followed by the arrival of European Crusaders who sacked the city, forced him to retreat.
In 1258, Niccolò and Maffeo Polo established an Assassins' Guild in Constantinople, after returning from the Assassin fortress of Masyaf, soon forming the Order's Turkish branch. They hid the five Masyaf Keys given to them by Altaïr in the Yerebatan Cistern, which could be entered by a secret door in Polo's old trading post, the Maiden's Tower, and beneath the Forum of the Ox, Galata Tower and what would become Topkapı Palace.
During the Renaissance, at some point between 1501 and 1507, the Doge of Venice, along with Sultan Bayezid II, sought to ally their considerable naval powers through a free trade treaty. However, the Templars were wary of any peace between the two, and became intent on interfering with their alliance. The Borgia family dispatched a force of mercenaries to disrupt the agreement, but they were quickly intercepted by members of the Italian Assassins, who set their ship aflame before they could depart.
By 1509, the Templars had began to relocate themselves to Constantinople due to their defeat in Italy and the disruption of their activities throughout Western Europe. The Templars formed a faction known as the Stewards of Byzantium and attempted to seize control of the city in the wake of Bayezid's absence, due to his civil war with his son Selim.
During this year, an earthquake uncovered one of the Masyaf Keys hidden beneath Topkapı Palace. Two years later, after traveling to Masyaf to research his Assassin heritage, Ezio Auditore da Firenze, Mentor of the Italian Assassins, arrived in Constantinople to retrieve the other Keys before the Templars. His leadership enabled the Assassins to reclaim their dens from the Byzantines and liberated most of the shops from their control, much as he had in Rome, as well as training several Assassins to the rank of Master Assassin through the assassination of key Templar agents. However, when Selim took control of the throne, he banished Ezio from Constantinople, though allowed him one final visit to the city to sort out his affairs, by request of his son Suleiman.
- Bernardo Baroncelli, one of the Pazzi conspirators, fled to Constantinople following the failed attempt to take over Florence.
- In Assassin's Creed: Revelations, the city was slightly smaller than Rome was in Assassin's Creed: Brotherhood, but it had more buildings, and was more densely populated.
- Alexandre Amancio, the Creative Director of the Assassin's Creed series, stated at E3 that Constantinople is a "really cool metaphor for Ezio meeting Altaïr", citing the fact that half of Constantinople was in Europe, and the other in Asia.
- It was also stated by Darby McDevitt that Constantinople would be a meaningful meeting of Ezio and Altaïr as the city itself was formerly under Christian control, then Muslim, which suited Altaïr and Ezio as the former hailed from a Muslim culture in Syria and the latter hailed from a Christian Italy.
- Constantinople was originally meant to be included in Assassin's Creed. 
- In Assassin's Creed: Revelations, it is only possible to explore the section of the city within the Constantinian Walls and Galata. The rest of the city, which is wholly inaccessible, can be seen from high locations such as top of the Galata Tower and the minarets of the Fatih Camii.