Castell Mawr, Trellech a'r Betws, is a subcircular enclosure, c.92-4m in diameter set on SW facing slopes towards a hill-summit, defined by a bank and ditch. It is generally reduced to a scarp, except on the SW, where a squared-off entrance facade has an additional bank.

(source Os495card; SN22NE4)
RCAHMW AP94-CS 1281
RCAHMW AP945150/44; 965048/44
J.Wiles 30.09.03