For many years, the small island of Sancti Petri was a restricted military zone, but it is now under the control of the Spanish Environment Ministry. The ruins of an eighteenth-century castle remain on the island; the tower of the castle has been substantially rebuilt, and it is now in use as a lighthouse. The name, Sancti Petri, also applies to the beach on the mainline, in Chiclana de la Frontera, directly opposite the island.

English Wikipedia article here:
http://en.wikipedia.org/wiki/Islote_de_Sancti_Petri

Spanish Wikipedia article on the castle here:
http://es.wikipedia.org/wiki/Castillo_de_Sancti_Petri