Occupying an area of 300 km², Castro Marim is a former military stronghold, nestling between two hills and overlooking the broad estuary of the river Guadiana marking the frontier with Spain. This largely rural borough is a stark contrast to coastal Montegordo. It consists of four freguesias (parishes): Azinhal, Odeleite, Castro Marim and Altura – the latter being the only one which has grown in recent years due to its coastal location. In the 1991 census, a total of 6,803 people were recorded as living in Castro Marim. Ten years later, there were only 6,593 – a drop of 3.1 per cent. Desertification of inland areas is a real issue in the Algarve as employment opportunities simply aren’t there for upcoming generations – nevertheless, from a visitor’s point of view, this is a breathtaking corner of the Algarve, full of natural beauty.

The area around Castro Marim is a protected natural reserve, rich in bird life. Its saltpans and marshes are feeding and breeding grounds for many species of water birds, including black-winged stilts, avocets, Kentish plovers and little terns. Flocks of several hundred flamingoes may be seen feeding in the pans out of breeding season. For directions and information about the species you can expect to see, ask at the office just inside the main gate of the famous castle within the village of Castro Marim.

The castle dates back to Roman times when the village held strategic importance due to its position, commanding views over neighbouring Spain. Its remains are open to the public. Visitors will discover that it is in fact a castle within a castle. The inner fortification is square with a cylindrical tower at each corner. In the 15th century, this was the headquarters of the Order of Christ, the religious-military order which succeeded the Knights Templar in Portugal. Prince Henry the Navigator was a Grand Master of the order. Later, the castle was greatly expanded. In the 17th century it was augmented by a second fortification, the Fort of São Sebastião, which stands on the other side of the village. The fort was built to strengthen Portugal's defences when it regained its independence after 60 years of Spanish subjugation between 1580 and 1640.