The main historical setting of Assassin's Creed: Revelations, Constantinople, is in modern-day Turkey.
Constantinople was the capital of the Byzantine Empire and is now known as Istanbul, Turkey. In the game's early sixteenth-century setting, the city is a major center of trade, politics, and cultural exchange. Ezio arrives there while pursuing clues connected to Altaïr Ibn-La'Ahad and the Masyaf keys.
The game uses the historical name Constantinople because that was the name commonly associated with the city during the period portrayed. Istanbul is the modern name, and the city officially adopted that name in the twentieth century. These two names refer to the same urban location, not to separate cities.
Revelations also includes the mountain fortress of Masyaf and sections set in Cappadocia. Those locations are geographically different from Constantinople, but they are connected through the game's narrative. The central city remains Constantinople, whose streets, districts, and waterways provide the main environment for Ezio's missions.