Which historical city is the main setting of Assassin's Creed Revelations?

Constantinople is the main historical setting of Assassin's Creed Revelations.

The game follows Ezio Auditore in the early sixteenth century as he travels to Constantinople, the capital of the Ottoman Empire. The city is divided into districts influenced by Ottoman, Byzantine, and other communities, giving the game a setting shaped by several cultural traditions.

Revelations also brings together Ezio's story and Altaïr Ibn-La'Ahad's legacy. Ezio searches for Altaïr's Masyaf keys, which contain memories of Altaïr and lead to the secrets beneath the former Assassin stronghold.

The city was officially renamed Istanbul in the twentieth century, but Constantinople is the historical name used for the setting in the game's period and in the game's title material. This is why “Istanbul” is not the expected answer to this historical-setting question.