Olcaf Castle, also known as Olocaf, is a fortress of Arab origin built in the 10th century atop a mountain 1,203 metres high, approximately 800 metres west of the urban centre of Olocau del Rey, in the Els Ports region. Its elevated location gave it enormous strategic value for controlling the surrounding territory. It was declared a Property of Cultural Interest on 25 June 1985, with code RI-51-0011001.

From an architectural standpoint, it is a medium-scale mountain construction with three non-concentric stepped enclosures and irregularly dispersed layout. Among the preserved remains are some sections of walls and the ruins of a cylindrical tower, testimony to its former defensive significance.

The fortress is linked to significant episodes of medieval peninsular history. According to Historia Roderici, the Cid Campeador, in service of the Zaragoza taifa, settled in this castle between 1088 and 1090 to control the area and spent a harsh winter there. For many scholars, Olocau corresponds to the "puerto de Alucant" cited in the Cantar de mío Cid, the Cid's third raiding camp chosen after the battle of Pinar de Tévar. In 1180, Alfonso II of Aragon granted ownership of the castle to the Order of the Hospital to encourage their participation in the Reconquest.

Among later historical episodes, it is notable that in the early 15th century, the castle held Margherita of Montferrat as a prisoner, mother of James of Urgell, who aspired to the throne left vacant by Martin the Humane. This fact illustrates the role the castle continued to play in Aragonese politics long after the Christian conquest.

Currently the castle lies in ruins and access is free. It is reached from the upper part of Olocau del Rey via a steep path of approximately 500 metres. From its remains, extensive panoramic views can be obtained over the Puertos de Morella and lands bordering Teruel province.