Masyaf is the Assassin Brotherhood's fortress in the original Assassin's Creed.
Masyaf is a settlement and stronghold in the Levant, serving as the headquarters of the Assassins led by Al Mualim. Altaïr returns there between missions to receive information, report progress and learn how the Brotherhood's internal hierarchy is changing. The fortress frames the game's historical storyline.
The location is based on the real Syrian mountain settlement of Masyaf, which was associated historically with the Nizari Ismailis. Assassin's Creed uses the place as a fictionalized headquarters within its broader historical setting. It is not the same as Alamut, another fortress frequently connected with popular accounts of medieval assassins.
Masyaf also becomes important in later games, especially Assassin's Creed Revelations, where Ezio seeks access to Altaïr's library. Monteriggioni, by contrast, is Ezio's Italian base in Assassin's Creed II and Brotherhood. Abstergo is a modern corporation, not an Assassin fortress.