In the original Assassin's Creed game, Altaïr's headquarters during the Third Crusade is Masyaf.
Masyaf is the mountain fortress that serves as the base of the Levantine Assassin Brotherhood in 1191. Altaïr returns there between major missions to receive instructions, report to Al Mualim, and recover from assignments. The settlement is presented as a fortified stronghold in the mountains of modern-day Syria.
The game also takes Altaïr to Jerusalem, Acre, and Damascus, which can make the headquarters easy to confuse with one of those larger mission cities. Those locations are targets or investigation areas, while Masyaf is the Brotherhood's home base. Historically, Masyaf is associated with the Nizari Ismaili community and a medieval castle, although the game combines historical material with fictional Assassin lore.
Masyaf later appears in the series through memories and references, reinforcing its importance to the early Brotherhood story.