Assassin's Creed Revelations uses Istanbul as the modern name for the city historically known as Constantinople.
The game's main historical setting is the Ottoman capital in the early sixteenth century. Ezio Auditore travels there in search of Masyaf keys connected to Altaïr's legacy and becomes involved in conflicts involving the Byzantine Templars and Ottoman political succession.
The city was known as Constantinople during the period depicted in the game. The name Istanbul became the standard international name later, especially after the Ottoman and Turkish eras. The game commonly calls the setting Constantinople in its historical narrative while the answer to the modern-name question is Istanbul.
This distinction matters because “Constantinople” and “Istanbul” refer to the same urban center, not two different cities. Bursa was an earlier Ottoman capital, but it is not the principal setting of Revelations.