In Assassin's Creed Revelations, the city represented as Constantinople is modern-day Istanbul.
The game is set mainly in Constantinople during the early sixteenth century, when the city was the capital of the Ottoman Empire. Ezio Auditore travels there to investigate the Masyaf keys and the legacy of Altaïr Ibn-La'Ahad, while the city provides districts, markets, underground passages, and major historical landmarks for exploration.
Constantinople was officially renamed Istanbul in the modern era, which explains why both names appear in discussions of the location. The game's historical setting uses Constantinople, not the modern political identity of Istanbul. This is separate from Assassin's Creed II, whose principal locations are Renaissance Italian cities.