It is divided into four levels with arcades, galleries, triforia and clerestories. The church has one of the finest triforia in all Basque Gothic Architecture. The longest pipes run horizontally across the North and South Triforia. Following Romanesque style, most window openings are fashioned as bi- and triforia. Masonry triforia are generally vaulted and separated from the central space by arcades. Still in the Gothic period, these triforia were replaced with large stained-glass windows.