Lair of Romulus
The Lairs of Romulus are secret locations featured in Assassin's Creed: Brotherhood that are similar to the Assassin's Tombs from Assassin's Creed II. Each lair houses a shrine to Romulus; each shrine contains a variety of a variety of treasures and a portion of a letter written by Marcus Junius Brutus. Attached to each letter is a key. There are six lairs in total, and obtaining all six keys will allow him to obtain the Armor of Brutus.
On the map of Rome, the Lairs of Romulus are marked by a black wolf symbol (representing the Followers of Romulus) and the actual Romulus treasure is marked on the map with an gold wolf symbol.
Lairs and locations
Palazzo Laterano - Inside one of the gardens.
Cloaca Maxima / Terme di Diocleziano (Note: Ezio must completely repair the Aqueduct that leads into the terme to access this, as the entrance is situated high up in the wall of the initially empty terme. However, the entrance can also be accessed without repairing the Aqueduct by dropping over the ledge and climb-leaping into the archway.)
Catacombe di Roma / Piramide Cestia.
Terme di Traiano
Colosseo - Ground level between the inner and outer walls.
Basilica di San Pietro - Outside, near the staircase that leads to the inner garden.
These lair missions are side quests, as obtaining the Armor of Brutus is not required to complete the main story of Assassin's Creed: Brotherhood. There is an exception, however; the Lair of Romulus mission at the Terme di Traiano is a core memory and must be completed to move forward in the main story.