In Assassin's Creed Mirage, Baghdad is the main medieval setting of Basim's story.
The game takes place in ninth-century Baghdad during the Islamic Golden Age, with the Abbasid capital serving as the hub for investigation, movement, and assassination missions. Basim begins as a street thief in the city of Anbar before joining the Hidden Ones and traveling to their headquarters at Alamut. He later returns to Baghdad as a trained Hidden One.
Mirage's depiction draws on the historical importance of Baghdad under the Abbasid Caliphate. The city was a major center of learning, commerce, administration, and culture. The game includes historical figures such as Caliph al-Mu'tasim and scholars connected to the period, while combining documented history with fictional characters and the series' Isu mythology.
Baghdad is not the same setting as Damascus in the original Assassin's Creed or Cairo in Assassin's Creed Origins. Those cities belong to different historical periods and protagonists, even though all three games feature Middle Eastern environments.