Ruins of an ancient strategic fortress located at the mouth of the Ulla River, in the Arousa estuary. Its main function was to defend the river access to Santiago de Compostela against incursions by Normans (Vikings) and Saracens. It was part of a broader defensive system organized by the monarchy and the bishops of Compostela. The medieval structure consisted of a double enclosure; the interior was a polygonal castle with four crenellated towers, flanking towers, and a courtyard with a central keep. Buildings attached to the walls housed kitchens, warehouses, etc. It included the Romanesque Chapel of Santiago (1122), attached to one of the towers. The outer enclosure served as a refuge and military parade ground. In the 18th century, the still-existing walls were used against an English attack (1719). Currently, the bases of two 9th-century towers and ruins of two others from the 12th century are preserved, as well as pre-Roman and Roman remains. The site is the annual stage for the Viking Pilgrimage, which recreates a Norman landing.