In Assassin's Creed Brotherhood, the Colosseum contains the entrance to one of the Followers of Romulus lairs in Rome.
The Followers of Romulus are a secretive cult that venerates the legendary founder of Rome. Ezio discovers their underground lairs while investigating clues scattered through the city. Completing these challenges reveals parts of a hidden message and rewards Ezio with the Armor of Brutus, a set associated with the Roman conspirator.
The Colosseum is one of Rome's most recognizable landmarks and a major navigational feature in Brotherhood. The game allows Ezio to climb and traverse the ancient amphitheater while pursuing missions, collecting items, and accessing its subterranean areas.
Brotherhood includes several other famous Roman monuments, including the Pantheon, Castel Sant'Angelo, and the Vatican. They are all important locations, but the Followers of Romulus content is divided among specific lairs, so the landmark named in the question identifies the relevant entrance rather than every cult location in Rome.