In Assassin's Creed Mirage, Baghdad is the principal setting for Basim's story.
Mirage takes place in ninth-century Baghdad during the Islamic Golden Age. The city is divided into districts that reflect different aspects of urban life, including commerce, administration, scholarship, and industry. Basim moves through these districts while investigating a hidden conspiracy and developing from a street thief into a member of the Hidden Ones.
The setting connects Mirage to the earlier appearance of Basim in Assassin's Creed Valhalla, but the game is set decades before Valhalla's main historical events. Baghdad's markets, rooftops, libraries, and political institutions support a more concentrated city-based design than the large regional maps of several recent entries.
Damascus is strongly associated with the original Assassin's Creed, while Cairo is a central city in Assassin's Creed Origins. Samarra was also an important Abbasid city, but it is not Mirage's principal setting. These neighboring historical associations make Baghdad the specific answer for Mirage.