This is the very heart of the Assembly House, the Casa de Juntas, where the Plenary Meetings of the General Assemblies of Bizkaia are held. An ancient institution, the origins of the General Assemblies date back to the Middle Ages, when meetings were held, as tradition demanded, in the open air around the Tree of Gernika.

Originally, a small hermitage (Santa María La Antigua) was to be found nearby, where a religious service was held prior to political events and where the act of swearing to respect the Old Laws (Fueros) also took place. With the passage of time, the General Assemblies moved their meetings into that small ecclesiastical enclosure.

From 1826 onwards, seeking premises more in accordance with the needs of the institution, the initial church was demolished to make way for the construction of the current building, for which the architect Antonio de Etxebarria took responsibility. Maintaining that tradition of close relationship between public life and religion, the Assembly Chamber was designed as a space capable of combining both functions.

We are, therefore, before a Church-Parliament. Although the ecclesiastical aspect has currently been relegated by politics, both the altar and the holy water fonts are still maintained as witnesses of bygone days.