This is one of the most important Gothic buildings in Bizkaia, particularly on account of its exterior elements which include buttresses, flying buttresses, pinnacles, etc. It has three naves with four sections each, culminating in a polygonal apse. The sections are crowned with simple rib vaults in the side naves, and a star-shaped rib vault in the centre. The ambulatory and chapels were added subsequently. It has a raked exterior due to the fact that the central nave is higher than the side naves. The façade is located at the foot of the church, and is decorated in two different styles. The main altarpiece is an example of the Hispano-Flemish sculpture conserved on the Iberian Peninsula.