Masyaf is the mountain fortress featured in the first Assassin's Creed game.
In Assassin's Creed, Masyaf serves as the headquarters of the Assassin Brotherhood and the home of its leader, Al Mualim. Altaïr returns there between missions to receive information, report to his master, and advance the game's story. The fortress is depicted as a settlement and stronghold in the Levant during the Third Crusade.
The real Masyaf is a town in western Syria, known historically for its medieval castle. It became associated with the Nizari Ismaili community, often called the Assassins in popular Western histories. The game uses this historical association while adding fictional characters and events.
Masyaf is not the same place as Alamut. Alamut was another major Nizari fortress, located in northern Iran, and it is frequently linked with the historical Assassins. In the game series, however, Masyaf is the distinctive headquarters connected with Altaïr's opening story.