Although much has been written about its purpose, there is no consensus among scholars. It served as a water cistern, but there are doubts as to whether this was its original function. The name because it is known today (in Latin "Domus Municipalis", in Portuguese "Casa Municipal") is due to the fact that it was used as a City Hall by the Municipal Administration of Bragança. In stone masonry, it presents a plan in the form of an irregular pentagon. It has an underground, which houses a vaulted cistern.