The only remains of the Castle of San Sebastián are the walls on the north side with two sentry boxes, since one bastion and two half bastions were demolished in order to build the Town Hall. It was built on a small hill, in the lower part of O Castro, to complete the bastioned area of the defences of the military area of Vigo. It was built in the 17th century by the military engineer Juan Villarroel y Prado and the brothers Carlos and Fernando de Grunemberg. It was built in order to defend the city of Vigo from English pirates, who were allied to Portugal in the Portuguese war of Restoration (1640-1668).