Assassin's Creed Brotherhood is primarily set in Rome. Ezio Auditore travels there to oppose the Borgia family and rebuild the Assassin Brotherhood.
Rome is substantially larger than the individual cities visited in Assassin's Creed II. Its districts include urban neighborhoods, countryside, landmarks, and the Vatican area. The game also uses the city to connect Renaissance politics with the series' fictional conflict between Assassins and Templars.
The Borgia-controlled city is initially divided into restricted areas. Ezio gradually liberates districts by defeating Borgia captains and restoring Assassin influence. He can then recruit and train new Assassins across Rome.
A common mix-up is Florence, which is strongly associated with Ezio's earlier story in Assassin's Creed II. Brotherhood's principal setting, however, is Rome, while later installments move the series to other historical cities and regions.