How do you enter the Lair of Romulus in Assassins Creed Brotherhood

If this is your first Lair of Romulus then do the following.. Open your map, look at the map legend. Once opened take a look at the icons for one that says "Lair of Romulus" once you know what it looks like you can find them on your map. When your at the location all of the Lair's entrances look like little building grates, small archways at the bottom of a building. There not that hard to spot really.

If you have found the first couple of them and there's no other icons on your map then you must complete Sequence 8 in order to unlock the rest of the Lairs.