Masyaf is Altaïr's main headquarters in the first Assassin's Creed game.
Altaïr Ibn-La'Ahad begins the game as an Assassin operating from Masyaf, a fortified mountain settlement in present-day Syria. The settlement serves as the home of the Assassin Brotherhood and is governed by its mentor, Al Mualim.
The game also sends Altaïr to three major cities in the Levant: Jerusalem, Damascus, and Acre. Those cities are the principal locations for his investigations and assassination missions, which is why they can be confused with his headquarters. Masyaf is different because it is the Brotherhood's central base rather than one of the main urban targets.
Historically, Masyaf is associated with the medieval Nizari Ismaili state and its fortress. The game uses that historical setting while adding fictional characters, secret technology, and an expanded portrayal of the Assassin order.