In Assassin's Creed, Altaïr Ibn-La'Ahad's main base during the Third Crusade is Masyaf.
Masyaf is the mountain fortress that serves as the headquarters of the Assassin Brotherhood in the first game. Altaïr returns there between major missions to receive instructions, report to Al Mualim, and advance the story. The settlement is presented as a fortified Syrian mountain stronghold rather than a large commercial city.
The game also takes Altaïr to Jerusalem, Acre, and Damascus, which can make the locations easy to confuse. Those cities are important mission areas, but they are not his central headquarters. Masyaf is especially significant because it establishes the Brotherhood's organization, its fortress-based structure, and Altaïr's relationship with his mentor.
Historically, Masyaf is a real settlement in western Syria associated with the Nizari Ismaili tradition. The game uses that historical setting while adding fictional characters, conspiracies, and supernatural technology from the wider Assassin's Creed narrative.