It is one of the most important Gothic buildings in Bizkaia (Biscay), above all for its exterior elements, buttresses, flying buttresses, pinnacles, etc. There are three naves with four sections each one, finishing with a polygonal apse.The sections have simple ogive vaults in the side naves and a starred ogive vault, in the central one.The ambulatory and chapels were added to it. From the outside it looks tiered since the central nave is higher than those at the sides. The main entrance is located at the foot of the church.The decoration of the main front is organised in two styles. The high reredos is one of the examples of the Hispano-Flemish sculpture.