Layers over layers. The history museum of Barcelona (Museu d'Història de Barcelona, MUHBA) shows 2000 years of city history in a very tangible way. The museum is in the Royal Palace which was built above the Episcopal palace which was built above the Roman city.
First you take a lift that brings you about 5 meters below street level. You are in an underground vault that is below the foundations of the upper buildings. The 4000 m2 vault shows the city wall, streets and factories/shops of the Roman times.