Only a very few poor traces remain of the old castle of Frenfano. Together with the remains of the castle of San Giorgio and of Castel di Onde they are the proof of the existence of a defence system to guard the routes in the Corno valley.

It stood above one of the ancient link roads between the area around Cascia and the Piano di S. Scolastica, at a point where people were forced to pass and which had already been fortified in ancient times.

In fact, near the fountain of Frenfano ruins and rough blocks can be seen sticking out of the ground, all belonging to an ogival, fortified enclosure 120 m. long and facing northeast. At the centre there is a macera, a fairly large mound of small blocks of limestone: it is probably all that remains of a tower. The castle is linked to other fortified enclosures in the area. Proof that this site was occupied even in prehistoric times can be seen in the archaeological finds of flint objects.

From Monte Frenfano there is one of the most beautiful views of the entire area around Cascia.