Twyn Castell is a castle mound or motte.
It is a sub-circular, steep-sided, flat-topped mound measuring 27m across at base and 18m (east-west) by 11m across the summit. It stands above steep slopes to the east, where it rises 6.6m high, but is only 3.2m high on the west where it faces level ground. There are no indications of masonry and no remains of a surrounding ditch or bailey. Erosion has exposed earth and large boulders in the mound.
The castle mound would have been crowned by a great timber-framed tower and would have been associated with a princely court or llys, presumably belonging to the princes of Senghenydd.
Sources:
Glamorgan Inventory vol.III part 1a, The Earlier Castles, p.70-1.
OS record card ST19NW12.

D.J.Percival, RCAHMW, May 1990.