The ruinous rectilinear stone walled castle enclosure, c.64m east to west by 40-60m, at Llanblethian Castle is set on a spur above steep slopes above the River Thaw. The encience, with a twin-towered gatehouse and two other towers on the east, and the entrance facade are thought to be early fourteenth century. A rectangular tower within is believed to be earlier. The castle is first mentioned 1234 and is believed to have been established around 1102. It was ruinous by 1741.

Source: RCAHMW 2000 Glamorgan III.1b 'the Later Castles', 166-185.

J.Wiles, RCAHMW, 14 January 2003