The House of General Assemblies, the seat of the highest institutional body of the province of Biscay, is located on the site of the mythical Oak of Gernika, symbol of Basque freedom and meeting place of all the General Assemblies since the Middle Ages until the construction of the new Neoclassical building between 1826 and 1833. The House of General Assemblies and the Oak of Gernika form a complex that has been declared a cultural heritage site.
The Plenary Hall is the heart of the building, with murals depicting the various former rulers of the province of Biscay. It houses an altar and a holy water font, as public and religious life used to be closely intertwined. The room with the glass roof was originally intended as an open courtyard and was later converted into a museum dedicated to the history of the Basque Country.