Its construction started at the beginning of the 12th century. it still conserves most of its original structure and Romanesque architectural elements. except for the massive external buttresses. which were added in the 18th century to reinforce it. Its beauty was rewarded when it was declared a National Monument in 1895. Consecrated by Archbishop Diego Xelmírez. its originality lies in the inclination of its walls and interior pillars. perhaps because it was built very close to the bed of the Sar River. which led to a gradual landslide and dragged the structure with it or. according to other theories. due to defects in the construction of the vaults. The interior possesses three naves which finish in apses. The main chapel is polygonal while the lateral ones are circular. The three are covered with barrel vaults which are reinforced with transverse arches. The ornamental motifs. typical of the Romanesque style. appear reflected in all the architectural elements found in its interior. This is not the case of its main façade. where only the lower part is conserved. since the rest was rebuilt in the 18th century. together with the exterior reinforcement mentioned above. The cloister still conserves a stretch of the original Romanesque style which coexists with the one made later. The old part is attributed to Master Mateo. author of the Pórtico de la Gloria. Many of the Romanesque elements which have been rescued are found in the Church's Museum.