La Seu derives from the Latin for "seat" and means, by derivation, a Cathedral, so, the Cathedral of Urgell. It houses early historical and more current historical memorials, including this for martyrs of the religious persecutions of the Spanish Civil War. It also is revered for its place in the martyrdom of the Cathars in the Pope's Albigensian Crusades in the nearby Languedoc, France, but spilling over boundaries into what is now Spain, see http://www.museuepiscopalvic.com/laprincesasavia/eng/ambit2.2.htm/. Religious paintings after that era tout the now unchallenged dogma that the Cathars had opposed so strongly.
Is this representing Saint Ermengol? He was Bishop of La Seu D'Urgell 1015-1035, and the Cathedral's patron saint. This is a cenotaph, not an actual burial place, I understand. There are six gilt panels, not all could fit, thus the two photos.
Changes in the saints du jour meant that earlier works were protected when overlays for the newly requested saints were imposed. For example, first Saint Catharine of Alexandria, martyred but whose eloquence converted notable pagans, and who was known as patron saint of scholasticism, see http://www.museuepiscopalvic.com/laprincesasavia/eng/ambit2.1.htm, then Saint Lucy.
Is this reliquary-casket related to her? There is reference to Which is which here I do not know. The dark-light coloration is striking, but could result from a pigment altering upon exposure? For discussion of a mural painting now more visible, not photographed here, See http://www.museuepiscopalvic.com/laprincesasavia/eng/ambit1.htm
Remains were often set in small caskets and set high above reach, affixed to the wall itself. Who is this?
The stone walls appear to have been covered with decorative frescoes, here well preserved. Click to enlarge. Were these part of the ornamentations destroyed by the Crusaders against the Cathars, see http://www.museuepiscopalvic.com/laprincesasavia/eng/ambit2.2.htm
Geometric and sunburst carving pattern, interior door. Note door is flat to the wall, no threshhold.