The Castillo de Santiago was built in the 15th century by the Dukes of Medina Sidonia, the powerful noble family who dominated this stretch of the Andalucían coast for centuries. It sits on a hill in the upper town, its tower visible from most of the old centre, and commands views across the river to the Doñana marshes.
The castle is restored and open to visitors. The interior has been cleared of later additions and you can climb the tower for panoramic views over the estuary and the Atlantic. The collection inside concentrates on the history of the Guzmán family — the Dukes of Medina Sidonia — who supplied Spain with its admiral for the Armada and held these lands for 600 years. The admission is modest and the visit takes about an hour.