Every Sunday morning, the City Hall will be opening to the public free of charge and allowing visitors to discover the history and architecture of the building. Guided tours will be offered in Catalan, Spanish and English and will take in the most important rooms. These are free visits and no prior registration is needed. An explanatory leaflet will be given to visitors upon arrival.

http://www.barcelona.cat/en/what-to-do-in-bcn/culture/detall/visits-to-the-city-hall_96208101011.html