The Serra del Montsec consists of a series of calcareous mountain ranges running roughly from east to west.

The highest peak, Santalís, is found in the central massif, standing at 1,678 metres, and dominates views over Àger and the plains of Lleida to the south and the Pyrenees to the north.

The southern face is virtually a great wall with a "step" about two-thirds of the way up, and the northern side, although by no means as steep, still stands at 1,000 metres above the Tremp basin below.

Montsec stands in a natural defensive position over the plains and is relatively protected from the icy winds from the north, and as such its strategic aspect was very important in times of the Reconquista.

In the 10th century, when the caliphate of Córdoba was at its height, Balaguer was the northernmost Arab city, and the castles of Castelló de Farfanya, Algerri, and Os de Balaguer were satellite fortifications which, together with a barrier of well-placed watch towers, formed the line of defense against the Christians to the north of Montsec.