Istanbul is the modern name of the city called Constantinople in Assassin's Creed: Revelations.
Revelations is set mainly in Constantinople during the early sixteenth century, when the city was the capital of the Ottoman Empire. The game follows Ezio Auditore as he searches for Altaïr's hidden library and investigates the conflict between the Assassins and Templars. Its districts include areas on both sides of the historic city and feature Ottoman landmarks and political factions.
Constantinople was renamed Istanbul in common international usage over time. The Ottoman government formally adopted Istanbul as the official name in the twentieth century, long after the period depicted in the game. Using the historical name Constantinople is therefore appropriate for the setting, while Istanbul identifies the same city today.
Alexandria is a major Egyptian city featured elsewhere in the franchise, especially Origins. Athens and Antioch are also ancient cities, but neither is the principal city of Revelations.