The Castle of Calatubo is a ruined medieval fortress on a rise outside Alcamo, close to the coast road. A village once stood near the castle, and the area was tied to the export of grain and millstones; even older traces include a necropolis dating to the 6th century BC.
The castle is not as polished as Alcamo's town-centre landmarks, but the setting and layers of history make it worthwhile if you are exploring western Sicily by car.