The first castle was built on a narrow rocky ridge above the town of Arén in the 9th century. A wall and two towers protected the south and west flanks of the castle. The mountain slopes steeply to the north. The best preserved is an archway. It belonged to a Romanesque church, whose colonnade, floor plan and semicircular apses on the back are easily recognizable. During excavations in 2004, a medieval necropolis with anthropomorphic graves was uncovered at this site.
The place still played an important role in the Middle Ages; in 823 it was mentioned as "Castro Arinio". Towards the end of the 19th century it lost importance.