Castro Marim provides a stark contrast to Monte Gordo and it nestles between two hills. Even though they are low hills they have a commanding view of the broad estuary of the river Guadiana, which marks the frontier between the Algarve and the Spanish province of Andalusia.
The larger of Castro Marim's two separate fortifications stands on the hill on the riverside of the road winding through the village. You can drive almost right up to the front gate and then walk around within the battlements. You will see that there is a castle within a castle. The inner one 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 that succeeded the Knights Templar in Portugal. 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).