In the original Assassin's Creed game, Al Mualim is based in Masyaf.
Masyaf is a fortified mountain settlement in modern-day Syria and serves as the headquarters of the Assassin Brotherhood during the game's historical storyline. Altaïr Ibn-La'Ahad returns there between missions to report to Al Mualim, the Brotherhood's leader and his mentor.
The other major cities in the game—Jerusalem, Acre, and Damascus—are mission locations rather than the Brotherhood's home base. This distinction is easy to miss because Altaïr travels repeatedly among all four places. Masyaf also appears in later Assassin's Creed stories, reinforcing its importance in the series' fictional history.
The game depicts Masyaf during the Third Crusade, while its real-world castle has a much longer history and is associated with the Nizari Ismailis. The game's secretive mountain headquarters combines that historical association with fictional characters, rituals, and technology.