This 16C fortress, converted into a Vauban-style citadel in the 17C, stands proudly, protected by its formidable high walls and sharp-edged bastions crowned with lookout turrets. It was a state prison prior to 1974, then an emergency refugee centre for those fleeing Angola. It looms over both the port to the east and the sea to the south. In the Capela Santa Bárbara there is a small museum that charts the history of the citadel.