Masyaf is the fortress and Assassin headquarters in Syria in Assassin's Creed.
In the game, Masyaf is the home of the Levantine Assassin Brotherhood and the base of its Mentor, Al Mualim. Altaïr returns there between missions to receive assignments, report on his progress, and interact with members of the Brotherhood. The fortress is presented as a major stronghold in the Syrian mountains during the Third Crusade.
Masyaf is also a real town in western Syria, known for a historic medieval castle. The game fictionalizes and dramatizes its role, connecting it to the historical Nizari Ismaili community often associated in popular culture with the medieval Assassins. It should not be confused with Alamut, another famous fortress linked to the historical Nizari Ismailis and used in later Assassin's Creed stories. Acre and Damascus are important cities visited during Altaïr's missions, but neither is the Brotherhood's main headquarters.