In Assassin's Creed, Altaïr's bureau at the start of the game is in Masyaf.
Masyaf is the mountain settlement that serves as the headquarters of the Assassin Brotherhood in the game's 1191 storyline. Altaïr Ibn-La'Ahad returns there after a failed mission, and the fortress becomes the setting for the opening punishment and later meetings with Al Mualim, the Brotherhood's mentor.
The location is based on the real Syrian town of Masyaf, which is associated with the Nizari Ismaili tradition and a medieval fortress. Assassin's Creed uses the historical setting as inspiration while creating a fictionalized story about Altaïr and the Templars.
A common mix-up is to name Jerusalem, Acre, or Damascus. Those cities are major investigation targets in the game, but Masyaf is Altaïr's headquarters and the place where the main story begins.