Castro Marim, four kilometres north of Monte Gordo, is a small town dominated by a medieval castle on a hilltop that gives 360-degree views across the Guadiana estuary, the saltmarshes, and into Andalusia. The castle dates from the 13th century and was the original headquarters of the Order of Christ — the successor to the Knights Templar in Portugal — before they relocated to Tomar.
The ruins are well-preserved, the walking is easy, and there are no crowds outside August. The view from the castle walls at sunset, with the river catching the light and Spain visible across the water, is one of the better free experiences in the eastern Algarve. A second fort, Fort São Sebastião, sits on an adjacent hill and can be visited on the same trip. From Monte Gordo, a taxi takes about ten minutes or you can walk along the riverbank path in under an hour.