Castelo de San Sebastián is a 17th-century fortress constructed in 1656 to defend Vigo from English pirates during Spain's war with Portugal. Built by military engineer Juan Villarroel y Prado and the brothers Carlos and Fernando de Grunemberg, the castle occupies a small island connected to the mainland by a narrow causeway on the lower slopes of O Castro. The structure originally featured bastioned walls with strategic vantage points overlooking the estuary. Though partially demolished to make room for the Town Hall, the remaining north walls with sentry boxes, historic tunnels used for troop movement, and an onsite museum preserve its heritage and offer panoramic views of Vigo's bay.
