It is the most important religious monument in Santander. Construction work began in the 14th century, and the three naves are incontrovertibly Gothic in style. Rebuilding has taken place over the intervening centuries at various times, most notably in the 17th century, and after the extremely damaging fire in 1941. The altar pieces in the cathedral's interior are impressive, as are some of the tombs, especially the sepulchre of the writer, Marcelino Menéndez Pelayo. The 15th century cloister runs along the southern wall and gives access to the interior. Rectangular in shape, its pointed arches are framed within half-pointed arcs. Admission is free; guided tours available during the summer months. Mass is conducted at: M-Sa 5p, 8p; Su 8:15a, 10a, 11a, 12:30p, 1:30p, 5p, 8p; visits during Mass not allowed.