In Assassin's Creed: Brotherhood, Rome is ruled by the Borgia family.
The game's main story takes place largely in Rome, where the Borgia family controls political, military and religious power. Ezio Auditore arrives after the events of Assassin's Creed II and works to weaken Borgia influence while rebuilding the Assassin Brotherhood.
Rodrigo Borgia appears as Pope Alexander VI, and his son Cesare Borgia serves as the principal human antagonist. The game fictionalizes their power struggle and places Ezio inside a broader campaign against Templar control.
Florence and Venice are central locations in Assassin's Creed II, which is why they can be mistaken for Brotherhood's principal city. Rome also appears in other series entries, including Assassin's Creed: Revelations, but Brotherhood is especially identified with Ezio's campaign there. The title's emphasis on recruiting and coordinating fellow Assassins reflects the city's role as the main strategic setting.