The castle is essentially two rings of wall climbing a low hill above the town. The outer wall dates from the reign of Afonso III in the 13th century; the inner keep is older, with foundations on a Moorish predecessor. Inside the walls almost nothing of the buildings survives — the great earthquake of 1755 flattened most of what was here — but the cisterns, the chapel ruins, and the perimeter walk are all intact.
The views are the real reason to climb up. From the ramparts you look down on the saltmarsh reserve, across the Guadiana to Ayamonte in Spain, and out to the Atlantic at Vila Real de Santo António. A small archaeology museum inside the walls displays finds from the site, including Islamic ceramics and the original keys to the town gates. Open daily except Mondays.