Ostia

Ostia was a major port and thoroughfare for the Italian city of Rome, situated on the Mediterranean coast. It was through this port that Cesare Borgia originally hoped to escape imprisonment in Castel Sant'Angelo, though he was captured by the Assassin Ezio Auditore before he was able to do so.

Following this, Bartolomeo d'Alviano and his mercenaries were positioned in the city to keep track of and prevent messages being passed between Cesare, who was now imprisoned in Spain, and the Borgia faithful who were still spread throughout Italy.

Reference

• Assassin's Creed: Brotherhood novel