This ancient ringfort, measuring roughly 55 metres at its largest dimension, consists of a subrectangular enclosure defined by collapsed stone walls. Rather than digging a defensive ditch, its builders took advantage of the natural drop of the rock face, letting the steep slopes provide protection where a fosse might typically be found.

The site's walls follow the irregular contours of the outcrop, creating an organic defensive perimeter that blends with the landscape. A possible entrance can be found on the southern side, whilst just outside the southwestern section stands an intriguing feature; a large, freestanding slab of rock reaching 1.25 metres in height. Whether this served as a territorial marker, had ritual significance, or fulfilled some practical purpose remains a mystery.

From its elevated position, Castleduff commands sweeping views across the surrounding countryside, a strategic advantage that would have been invaluable to its inhabitants. Like many cashels throughout Ireland, it likely served as a fortified homestead during the early medieval period, providing both security and status to a farming family or local chieftain. Today, though its walls have tumbled with time, the site remains a prominent landmark in the Donegal landscape, offering visitors a tangible connection to the region's ancient past.