Constantinople is the main city Ezio Auditore visits 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 travels there while searching for the keys to Altaïr Ibn-La'Ahad's hidden library in Masyaf. The city campaign includes districts on both sides of the Golden Horn and the nearby Cappadocian underground settlement.
Ezio works with members of the Ottoman Assassin Brotherhood, including Yusuf Tazim, and becomes involved in the succession struggle surrounding Sultan Bayezid II. The setting gives the game a distinct Ottoman architectural and political background compared with the Italian locations of Assassin's Creed II and Brotherhood.
Constantinople was officially renamed Istanbul in the twentieth century, but Constantinople is the historical name used for the game's setting. That difference explains why modern references may use Istanbul even though the game and its period commonly identify the city as Constantinople.