In Assassin's Creed: Brotherhood, almost the entire main map is Rome.
Brotherhood follows Ezio Auditore after Assassin's Creed II and places him primarily in Renaissance Rome. Ezio works to weaken the Borgia family, liberate districts, recruit and train new Assassins, and rebuild the Brotherhood's influence in the city.
Rome was the political center of the Papal States and contained landmarks such as the Colosseum, the Pantheon, and the Vatican area. The game compresses geography and chronology for playability, but many streets and monuments are recognizable versions of real places.
Brotherhood also includes brief sequences outside the main city, including Monteriggioni and modern-day locations. Those exceptions do not change the answer: Rome is the central open-world setting. Constantinople is the main city of Revelations, and Paris is the principal city of Unity.