Castell Vell, also known as Fadrell Castle, Magdalena Castle, Moros Castle or Sas Castle, is one of the most important fortifications of Andalusian origin in the Valencian Community. It stands on the hill of la Magdalena, in the municipal area of Castellón de la Plana, approximately 7 kilometres from the current urban centre. Its Islamic origins date back to the 10th and 11th centuries, and it constituted the primitive nucleus around which the first settlement formed that would give rise to the city of Castellón.

The site has exceptional historical relevance: the hill on which it stands was the place where the population settled before, in the 13th century, Jaume I authorised the relocation of the inhabitants to the plain, giving rise to present-day Castellón de la Plana. The Poem of the Cid places Fadrell Castle historically at the end of the 11th century as an important Muslim stronghold in the region.

The preserved remains include fragments of walls and structures typical of Islamic defensive architecture, although the complex is in a state of partial ruin. Archaeological excavations carried out at the site have made it possible to document materials and structures corresponding to different phases of occupation, making it also a place of archaeological interest.

Access is via the Camino del Desert, in the vicinity of the Magdalena hill. The natural surroundings of the hill offer panoramic views over the Castellón coastal plain and the Mediterranean Sea, and form part of the Route of the Cid as it passes through Castellón.

Castell Vell is a protected heritage site and constitutes one of the most eloquent vestiges of Islamic presence in the north of Castellón province, as well as of the urban formation process of the provincial capital during the Middle Ages.