There is no fosse, the encircling ditch that typically accompanies a ringfort's earthen bank, and no recognisable entrance survives. It sits on a south-facing terrace above a stream, an oval platform roughly forty metres by thirty-two, and its boundaries are only partly the work of human hands.

A rath is an early medieval farmstead enclosure, usually dating from roughly the fifth to the twelfth century, built to define and defend a household's space rather than to serve any military purpose. At Castlebaun, the enclosure is formed by a combination of constructed and natural features. A low bank of earth and stone, around four metres wide and no more than half a metre high, curves around the western and north-western sides. A steeper scarp, rising between about eighty centimetres and one and a half metres, takes over along the northern and north-eastern arc and again from the south-east round to the south-west. Elsewhere, the natural slope of the ground does the work entirely, so that the monument blurs at its edges into the landscape rather than announcing itself with a clean, dug boundary. The absence of a fosse is notable; it may mean the builders relied on the terrain itself to compensate, or that any ditch was never cut in the first place, perhaps because the slope made it unnecessary.