Assassin's Creed Revelations is primarily set in 1511.
The historical portion of Revelations follows an older Ezio Auditore in Constantinople, where he searches for keys connected to Altaïr Ibn-La'Ahad's former library in Masyaf. The game therefore links the lives of two major protagonists across different centuries.
The Constantinople storyline takes place in the early sixteenth century, during the Ottoman succession struggle involving princes Selim and Ahmet. The city is represented through districts on both the European and Asian sides of the Bosporus, and Ezio works with local Assassins led by Yusuf Tazim.
The year 1511 is sometimes confused with the dates of the fall of Constantinople in 1453 or Columbus's voyage in 1492. Those events belong to earlier history and are not the primary date of Ezio's final major adventure.