The two castles dominate everything. The medieval Castelo de Castro Marim sits on the higher of two hills above town, with the smaller 17th-century Forte de São Sebastião opposite — together they form one of the more unusual defensive arrangements in southern Portugal. Both are open to visitors and the views across the salt flats and Guadiana River into Spain are the main reward.
Below them stretches the Reserva Natural do Sapal, a vast wetland that is one of the best birdwatching sites on the Iberian Peninsula. Flamingos, white storks, black-winged stilts and avocets are all regular. The visitor centre at Cerro do Bufo has hides and a few easy walking trails out among the salt pans.