The oldest part of the castle is the eastern tower, built of tapia masonry on the steep slope of the cliff. It dates from the Almohad period, i.e., the late 12th to early 13th century. The second tower of the complex probably dates from the first decades of the 14th century. Originally a Moorish foundation, after the Reconquista the castle was located in the border area between the Kingdom of Murcia, claimed by Castile, and the Kingdom of Valencia, conquered by Aragon. A chain of castles was built in this area, mainly along the Vinalopó River. Around 1240, the castle was wrested from the Muslims under Aragonese leadership and with the help of the Templars and the Order of St. John.
The castle in Sax is located on the CidCT route: Camino del Cid Cyclotourista route of castles in the historically contested border region between the Moors and the kingdoms of Castile, Valencia, and Aragon.
Under the name "El Cid," Spain's knight Rodrigo Diaz de Vivar became a symbolic figure of the Reconquista. The Castilian nobleman fought against Moorish rulers and Christian rivals in the 11th century. He never lost a single battle; the Moors respectfully called him Alsayyed (Lord of the Leaders), which later became "El Cid." The Spanish, however, always referred to him as "El Compeador" (The Warrior).