Masyaf is the main headquarters of the Assassin Brotherhood in the original Assassin's Creed.
Altaïr Ibn-La'Ahad returns to Masyaf after a failed mission at the beginning of the game. The mountain settlement is led by Al Mualim, the Brotherhood's Mentor, and serves as the place where Altaïr receives missions, learns of political developments, and returns between investigations.
The game also sends Altaïr to the major cities of Jerusalem, Acre, and Damascus. Those cities contain the targets and investigations that drive the campaign, but Masyaf remains the Brotherhood's home base and the starting point for its Levantine operations.
Masyaf is a real settlement in present-day Syria, associated with a medieval fortress. The game uses its historical setting while adapting the secretive Assassin order into the series' fictional conflict with the Templars and the Pieces of Eden.