This castle stands in a remarkable site that was already inhabited in Roman times. It was built in 1135 by the first king of Portugal, Afonso Henriques, to defend the southern border of his kingdom when Santarém and Lisbon were under Moorish rule. After the liberation of these towns in 1147, the castle lost its importance. In the 14C, King Dinis had it rebuilt so he could live there with his wife, Queen Isabella of Aragon.