In the original Assassin's Creed, Masyaf is the headquarters of the Assassin Bureau where Altaïr reports to Al Mualim.
Masyaf is a fortified mountain settlement in present-day Syria and serves as the Assassins' main base during the game's 1191 storyline. Altaïr Ibn-La'Ahad returns there between investigations to receive instructions, improve equipment, and report his progress.
The game also visits Jerusalem, Acre, and Damascus, which are major investigation locations rather than the headquarters. This distinction is a common source of confusion because Altaïr spends much of the story travelling between those cities. Masyaf is also associated with the historical Nizari Ismaili leader Rashid ad-Din Sinan, whose fictionalized counterpart is Al Mualim.