In the original Assassin's Creed game, Altaïr's main headquarters is Masyaf. The mountain settlement serves as the headquarters of the Levantine Assassins and is where Altaïr receives missions, reports to Al Mualim, and returns after major assignments.
Masyaf is a fictionalized version of the real Syrian fortress town of the same name. The historical Masyaf was associated with the Nizari Ismaili state, a medieval community often linked in popular culture with the word “assassin.” The game adapts that setting into a dramatic mountain stronghold.
Players can visit several major cities during the game, including Jerusalem, Acre, and Damascus. Those cities are mission locations rather than Altaïr's permanent base, which is why they are frequent mix-ups. The village of Arsuf is another important location, but it is not the Brotherhood's headquarters.
Masyaf also remains important in later series lore. It represents the early Levantine Brotherhood and the setting in which Altaïr's leadership, reforms, and use of the Apple of Eden reshape the order.