Castell Carno Hillfort is a roughly oval enclosure measuring circa 68m east to west by 45m. It occupies the end of the spur overlooking Carno and it is defined by a bank ditch and counterscarp on the north and north-west and elsewhere by scarps above steep natural slopes. A third bank lies beneath the field boundary to the west. The possible entrance gap, which is 2.7m wide, in the west may be modern and there are no convincing signs of earthworks in wood. The site was visited by the RCAHMW on 19 July 1910.

Claire Parry, RCAHMW, 11 August 2011.

Sources:
Inventory of the Ancient Monuments in Wales and Monmouth I - County of Montgomery , Royal Commission on the Ancient and Historical Monuments of Wales , 1911, no 43
Clwyd-Powys Archaeological Trust Historic Environment Record