A quadrant of double scarps, c.18m apart, define an enclosure, c.68m by 76m, otherwise resting in an angle of steep natural slopes. (Coflein as Iron Age defended enclosure)
Multivallate sub triangular spur end enclosure. some 120m across. inner enclosure abutts N facing scarp crest of E-W spur with second (and poss third-Hughes, I.T., 1928) rampart to W. whole damaged by small quarries. (Clwyd Powys Archaeological Trust HER)
Until the 1960s this was the suggested site for Hubert's Folly, mentioned as a new castle in 1228, but this was established as Old Hall Camp whilst Pen y castle is probably an eroded bivallate hill-fort.