In the original Assassin's Creed, Masyaf is Altaïr Ibn-La'Ahad's headquarters.
Masyaf is the mountain stronghold of the Levantine Assassin Brotherhood and the home of its leader, Al Mualim. Altaïr returns there between missions to receive instructions, report his progress, and learn more about the political conflict surrounding the Crusades. The settlement is also the location of the Brotherhood's main fortress.
The game sends Altaïr to three major cities: Jerusalem, Acre, and Damascus. Those cities contain the investigation and assassination targets, while Masyaf functions as the central base from which the missions begin and to which Altaïr returns. This distinction is important because the three target cities are often remembered more prominently.
The historical Masyaf is a real settlement in modern Syria, associated with the medieval Nizari Ismaili community. The game uses the location as part of its fictionalized account of the Assassins. Altaïr's story is set during the Third Crusade, while the modern-day framing follows Desmond Miles.