The defended enclosure at Coed Pentwyn is a large, oval, bivallate enclosure standing at about 305m above OD at the south-east end of a small, gently undulating ridge separated by the small valley of Cwm Onneu fach from the imposing cliffs of Craig y Ciliau to the south. The east and south sides have steep natural defences but on the north the site is easily approached and it is here that the strongest fortifications are found. The defences survive as substantial, though denuded, earthworks and are covered with grass, bracken and trees. The site measures internally at least 140m north-west to south-east by 103m, an area of 0.89 hectare. The main rampart, constructed largely of stone, is most prominent around the north where its inner and outer scarps are respectively 1.7m and 4.2m high. Around the south it has been reduced to a single outward-facing scarp. No revetment is visible. The outer bank is concentric with the inner terminating on the north where the latter inturns sharply to form the north-west side of the entrance. Around the north the rampart rises to 3.3m high externally, becoming less prominent westwards. On the west it is a broad level-topped embankment, apparently slighted, its inner and outer faces respectively 1.5m and 1.3m high. The outer rampart is ditched, on its west side only, for a short length and with counterscarp bank up to 0.8m high. Whether or not both inner and outer defences were built together is unclear, though it appears that attempts at strengthening the west fortifications were left unfinished. The inturned entrance banks formed a passage beween them, now blocked with rubble, which can be followed for 2m to the south-west as a faint depression. A possible hut site represented by a round hollow lies in the angle formed by natural scarps on the south-east. Here, steep natural slopes form the only visible defences though it is likely some artificial work completed the perimeter.
Ploughing has damaged the south-east sloping interior and has spread badly the west counterscarp bank, while the west outer bank seems to have been slighted and levelled deliberately. Ruinous stone structures have been built into the earthworks on the north-west and north-east along with a later field system of stone walls utilising some of the lines of the defences.
Source:
RCAHMW, Brecknock Inventory vol.I, part ii (1986), p.78.

RCAHMW, 29 January 2016