This frontier castle is Arab in origin and was reinforced and extended by the Christians from the time of its conquest. It contains a unique element in Hispanic Arab architecture.
It was built in the 12th century by the Almohads, and reinforced from the mid-13th century onwards due to its location on the frontier between the kingdoms of Aragon and Castile. It is today in a good state of conservation, and comprises a double enclosure. The exterior, which follows the shape of the hill overlooking the village of Biar, has four semi-cylindrical towers at the corners, all with battlements and arrow slits. The interior also has three semi-cylindrical towers, although its most outstanding feature is the shell keep. This building has a rectangular floor plan, 17 metres high, with a ground floor and two other floors. Its peculiarity is to be found on the first floor, which is covered by a traditional Almohad ribbed vault, the only example of its kind in Spanish military architecture.