In Assassin's Creed Mirage, Baghdad is Basim's home and the main setting.
The game takes place in ninth-century Baghdad, during the Islamic Golden Age under the Abbasid Caliphate. Basim begins as a street thief in the city before traveling to Alamut and joining the Hidden Ones, the organization that precedes the later Assassin Brotherhood.
Baghdad was founded in 762 by the Abbasid caliph al-Mansur and became a major center of government, scholarship, medicine, literature, and trade. Mirage uses that historical setting for a fictional story involving Basim, the Hidden Ones, and the Order.
Mirage is set before Assassin's Creed Valhalla, where Basim is introduced as an older character. Baghdad is distinct from Alamut: Baghdad is the principal urban setting, while Alamut is the mountain fortress and training center associated with the Hidden Ones in the game.