At Llangynwyd the remains of a once splendid medieval fortress are now reduced to scanty ruins and earthworks. The castle was the centre for the upland lordship of the lords of Glamorgan in Gorfynydd cantref, thought to have been annexed by 1147. It is first mentioned in 1246, but is thought to be a twelfth century foundation. The castle was devastated in 1257 and was subsequently rebuilt. It was burned in the riots of 1294-5 and does not appear to have been restored. The site was partially excavated in 1906.
The castle occupies the tip of a steep sided spur between two streams and consists of a heart shaped walled inner court some 35-37m across set at the south-east end of a larger outer court. The inner court had a mighty rock-cut ditch, except on the north-east, with a great counterscarp bank on the west and south sides. The outer court is some 80-90m deep and was enclosed by a 120m arc of rampart and ditch. The wall of the inner court is quite poorly constructed and may pre-date the devastation of 1257. A great twin-towered gatehouse facing into the outer court was probably added in the 1260s and has been compared to the great gatehouse at Caerphilly Castle. A D-shaped tower, built in a similar style, was added a little to the north of the gatehouse.
Source: RCAHMW Glamorgan Inventory III.1a The Earlier Castles (1991), 258-63
John Wiles, RCAHMW, 13 February 2008