Far fewer notice that the rocky knoll on which the castle sits contains something rather older and stranger than any medieval battlement: a natural cave, locally known as the Dungeon, that burrows 17.5 metres into the northern face of the hill through a passage so low and narrow that moving through it requires some determination.

What makes the cave particularly interesting is not its geology alone but the way it was folded into human architecture. When the MacCarthy family built their manor-house on the site in the 17th century, they did not simply ignore the cave or seal it off. Instead, they incorporated it directly into a gun turret of the new building. The outer 3.5 metres of the natural passage were artificially widened and heightened to make the entrance more workable, and a section of drystone walling, the same length, was raised against the enlarged cave mouth. Drystone construction uses no mortar, the stones being carefully laid so that their own weight holds them in place, and its use here suggests a practical, expedient approach to integrating a geological oddity into a defensive structure. The result is a space that is neither entirely natural nor entirely built, the work of water and time at one end, and the work of MacCarthy masons at the other.