In Assassin's Creed Revelations, the real historical city called Constantinople in the game is Istanbul. The story takes place in the early sixteenth century, when the city was the Ottoman Empire's capital and was commonly known in Europe as Constantinople.
Ezio Auditore travels there while searching for Altaïr Ibn-La'Ahad's hidden library and the keys needed to open it. The game depicts districts on both sides of the Bosporus and includes Ottoman figures such as Prince Suleiman, who later became Sultan Suleiman the Magnificent.
The name change is a frequent source of confusion. Constantinople was the city's longstanding historical name, while Istanbul became the official name of the Turkish capital in the twentieth century. In the game's period, using Constantinople is historically appropriate, even though modern maps identify the city as Istanbul.