15th century Gothic building. It has an irregular plan with four naves and a cloister from the 16th century, topped with an ogive vault. The central nave is higher than the side ones, and is supported outside by on the exterior by flying buttresses.The naves finish in the apse.There is no ambulatory or abbatial chapels.The crypt is under the altar and dates from the 17th century. Here lies the tomb of the Counts of Oñati, built in alabaster. The cloister was built over the river and is supported on top of two bridges with one span. At the foot of the church the tower rises up, Neoclassic in style with three sections.The main front is Baroque, decorated with reliefs.