In the original Assassin's Creed (2007), Altaïr's Assassin headquarters is located in Masyaf. The mountain settlement is presented as the central stronghold of the Levantine Assassins, led by Al Mualim during the game's twelfth-century storyline.
Masyaf is a real Syrian town associated with a medieval fortress. In the game, its elevated location and fortified architecture make it a natural base for training, receiving missions, and returning after investigations in other cities. Altaïr begins and ends many important stages of his campaign there.
The other major locations are Jerusalem, Acre, and Damascus. Each is a separate city where Altaïr tracks targets and gathers information, so players sometimes confuse one of those mission destinations with the order's headquarters. Masyaf is also distinct from Alamut, the historically famous fortress often linked with the Nizari Ismailis.
The game's modern-day framing story follows Desmond Miles, but the question concerns Altaïr's historical setting in the Third Crusade. Masyaf therefore anchors both the narrative and the game's recurring travel structure.