In the original Assassin's Creed, Masyaf is the Assassin bureau where Altaïr begins his missions.
Masyaf is a settlement in present-day Syria and serves as the headquarters of the Assassin Brotherhood in the game's 1191 storyline. Altaïr Ibn-La'Ahad returns there after the opening mission, and the fortress is where he receives assignments from Al Mualim, the Brotherhood's mentor.
The game's other major Syrian and Holy Land locations include Damascus, Acre, and Jerusalem. Those cities are mission destinations rather than the Brotherhood's main headquarters. Masyaf is therefore easy to confuse with the target cities, especially because the player repeatedly travels between them.
Historically, Masyaf was associated with the Nizari Ismaili community and its fortifications. Assassin's Creed uses that historical association as a foundation while adding fictional characters, conspiracies, and technology to the setting.