Istanbul is the modern name for the city called Constantinople in Assassin's Creed: Revelations.
The game is set primarily in Constantinople during the early sixteenth century, when the city was the capital of the Ottoman Empire. Ezio Auditore travels there to search for keys connected to Altaïr's former headquarters and to investigate the conflict among Assassins, Templars, and Ottoman political factions.
Constantinople was the historical name used in the game's period. The city was officially renamed Istanbul in the twentieth century, although “Istanbul” also appeared in historical usage before that change. Using both names helps explain why sources describe the same location differently.
Revelations includes districts on both sides of the Golden Horn, along with locations such as Hagia Sophia and the Topkapı area. The game also concludes Ezio's main trilogy and includes playable sequences featuring Altaïr Ibn-La'Ahad.