The building is really a museum. La Pedrera is another of Gaudi's creations and is the second most visited museum after La Sagrada Familia.
The quarry has to be one of the most unusual apartment buildings ever built. At the time potential tenants were reluctant to rent the rooms because the walls were irregularly shaped and were concerned that no furniture would fit. Today La Pedrera is one of the most important museums in Barcelona.
Here is a picture looking out at Barcelona from the rooftop.