In the original Assassin's Creed, Altaïr's headquarters during the Crusades is Masyaf.
Masyaf is a fortified mountain settlement in present-day Syria and serves as the main base of the Assassin Brotherhood in the game. Altaïr returns there between missions to receive instructions, report progress, and meet Al Mualim, the Brotherhood's leader.
The game is set during the Third Crusade, with major missions taking place in cities including Jerusalem, Acre, and Damascus. Those cities are targets of investigation and assassination, not Altaïr's permanent headquarters. Masyaf therefore has a different role from the urban locations visited during the story.
The fictionalized game version draws on the real medieval fortress of Masyaf, which is associated with the Nizari Ismaili community. Assassin's Creed combines this historical setting with supernatural lore and science-fiction framing.