Atop the highest hill of the region stands the Alcácer do Sal Castle. Once nearly impenetrable, the imposing fortress which once had more than thirty 25 m towers, now houses the Pousada D. Afonso II and the Archaeological Crypt, an underground museum.

Surrounded by two ramparts, the Alcácer do Sal Castle is a rare example of military constructions made of topsoil. Archaeological digs built in the vicinity of the fortress found remains from the Neolithic, Iron Age and Roman and Arab occupations. Some of the finds can be seen at the museum currently housed in the castle.

Starting in the 19th century, when the building was abandoned, the building fell into disrepair, until the 1990s, when extensive restoration work was carried out on the convent and the castle. In 1998, the old convent was transformed into a Pousada and ten years later, the Archaeological Crypt of the Alcácer do Sal Castle was opened.