The fort was constructed on the site of an ancient watchtower and construction began in 1556 under the direction King John III who wished to have the mouth of the Tagus river sufficiently protected from invaders.

The fortress was used as a political prison both at the end of the Portuguese monarchy and during the Salazar dictatorship with prisoners held in awful conditions.

The structure was constructed primary for defence and lacks any artistic beauty with the solid walls constructed from drab stone and masonry plaster.