Rome is the primary open-world setting for Ezio Auditore in Assassin's Creed: Brotherhood.
The game continues Ezio's story after Assassin's Creed II and focuses on his effort to weaken the Borgia family in Renaissance Italy. Rome contains districts controlled by the Borgia, and Ezio can liberate them, restore shops, and recruit members for the Brotherhood.
Florence and Venice are central locations in Assassin's Creed II, so they are frequent alternatives in this question. Brotherhood does revisit other places during specific sequences, but Rome is the game's defining city and the location where most of its open-world activities take place.