The Castle of Castro Marim crowns a low hill about 5 km north of Vila Real de Santo António, visible for miles around across the flat estuary country. The ruins are medieval in core but were heavily rebuilt in the 17th century when Portugal feared a Spanish incursion from across the Guadiana. The circuit of walls is largely intact and the views from the battlements take in the river, the salt pans, the sea, and across into Spain.
The castle is open daily and cheap to enter. Inside, a small museum covers the Order of Christ and the castle's role in the Reconquista. The ruins of a Gothic church of São Tiago survive within the walls. The village of Castro Marim below the castle is quiet and pleasant, with a few cafés and restaurants. Combined with the marsh nature reserve immediately below the hill, this makes an easy half-day from Vila Real.