The downstairs area is basicly a hallway with different rooms branching off to the sides. The enterence to the catacombs far beneath the Thieves HQ is hidden somewhere downstairs, but very few know where that is located for certain reasons.
The doors on the right wall lead, in order, to the storage bay for liquor, the armory, the jewlery storage bay, Bane's Room and Merek's room. The doors on the left wall are just spare rooms, with the exception of Jodd's room.