Castell Mawr is situated some 220m south-east of medieval St Brynach's Church (NPRN 310033). It comprises earthworks of a medieval motte and bailey castle, laid out along the crest of a north-west to south-east ridge rising above a stream confluence. The ditched motte is a subcircular steep-sided mound, 36m in diameter and rising some 2.3m to a summit area 14.5-16m across, and has a subrectangular court, or bailey, some 46m north-south by 30m, defined by scarps, attached on the west.

J.Wiles 29.03.05