A headland jutting into open water provides natural defences on three sides, and the builder need only close off the landward approach with a bank or ditch. At Castlewidenham in north Cork, the same logic has been applied entirely inland, using a sharp bend in the river Awbeg to create a triangular spit of high ground defended by steep, wooded cliffs on every side but one. It is an elegant piece of territorial thinking, and it worked well enough that later builders kept coming back to the same spot.

The fort encloses a substantial area, roughly 300 metres along its longest axis and up to 200 metres wide, with the pointed end of the triangle facing south-east towards the river bend. The landward gap was sealed by a ditch, still traceable on the ground, running about 90 metres north from the southern cliff before turning north-north-east for a further 125 metres to meet the northern cliff face. The ditch is approximately 5.4 metres wide and 0.4 metres deep where it survives most clearly; the northern stretch has largely flattened to a low scarped edge. The pre-Norman name for the place was Dún Cruadha, according to Power writing in 1932, and Ó Murchadha has identified it as a stronghold of the Uí Laoghaire, the Gaelic dynasty who controlled this stretch of north Cork before the Norman settlement of the twelfth century. What gives the site an unusual layered quality is what came afterwards: the outer bawn wall of a later medieval castle, a bawn being an enclosing courtyard wall typically associated with tower houses and fortified manors, appears to follow the line of the earlier fort's defences almost exactly. Whoever built that castle was either making deliberate use of an already-fortified perimeter or found that the natural geography simply imposed the same solution twice.