One of the most important examples of Romanesque architecture in Spain. It was built in the 12th century, on the initiative of the Count Martín Fernández, on top of an old chapel that was devoted to St. Peter. It has a rectangular floor plan with three naves and three apses. A remarkable feature of its exterior is the Romanesque south portal, decorated with archivolts. In one of its niches we see a relief that depicts St. Martin sharing his tunic with a pauper. Inside, the choir stands out, as well as the numerous tombs (including that of Diego de Santiesteban) and the high reredos, made by Churriguera in 1731.