In Assassin's Creed: Brotherhood, Rome is the main setting for Ezio's fight against the Borgia.
The game continues Ezio Auditore's story after Assassin's Creed II and places him in Rome, the center of Borgia power. Ezio works to weaken the Borgia family, liberate districts, recruit new Assassins, and rebuild the local brotherhood. The city functions as a large interconnected open-world environment rather than a collection of separate city missions.
Rodrigo Borgia, now Pope Alexander VI, and his son Cesare are the principal Borgia figures in the narrative. Their political and military power gives Ezio a different kind of opponent from the more localized conspiracies of the earlier game.
Florence and Venice are major settings in Assassin's Creed II, while Forlì also appears in the broader Ezio-era story. Those locations are easy to associate with Ezio, but Brotherhood is specifically identified with Rome. The Colosseum, the Vatican area, and the rebuilt guild headquarters are among its recognizable locations.