The castle sits on a low basalt hill above the old part of town, its silhouette of ruined towers and curtain walls visible from most of the surrounding valley. The Moors built it in the 10th century as part of a chain of defensive positions along the Atlantic coast, and the Portuguese have left it more or less alone since 1249 — which means there is not much to see inside beyond the walls and the cistern, but the views are the point.
From the top you look out over the Aljezur River valley, the town spread below you in two pieces (the old Moorish quarter on the slope, the newer town in the flat below), and the hills of the natural park rolling away to the coast. The castle is freely accessible at all hours and takes about twenty minutes to explore at a stroll.