Castro Marim's huge castle has an intriguing borderland history. Much of the area was destroyed in the 1755 earthquake, but the ruins of the main fort are still impressive. Inside the evocative, derelict castle walls stands a 14th-century church, the Igreja de Santiago, where Prince Henry the Navigator is said to have prayed. A small museum displays local artefacts dating back to the Iron Age. Most unforgettable are the views across the salt flats and into Spain.
In the 13th century, Dom Afonso III built this castle over Moorish and Roman foundations in a dramatic and strategic position for spying on the Spanish frontier. In 1319 it became the first headquarters of the religious military order known as the Order of Christ, formerly the Knights Templar. Until they moved to Tomar in 1334, they used this castle to keep watch over the estuary of the Rio Guadiana and Spain.
Most of the grand stretch of ruins today, however, dates from the 17th century, when Dom João IV ordered the addition of vast ramparts. At the same time, Forte de São Sebastião, a smaller fort (closed to the public), was built on a nearby hilltop.
A medieval fair takes place in and around the castle over the last weekend in August.