Llantrisant castle is a magnificent towered medieval fortress now reduced to fragments. It forms a group with the church (NPRN 14034) and borough (NPRN 33133), all set in a saddle between two hills atop the escarpment marking the southern side of the Glamorgan uplands.

A Norman earthwork and timber castle may have stood on this small steep-sided spur next to the grand twelfth century parish church, but the present castle was built in 1246 by the lords of Glamorgan following their annexation of upland Penychen. It was the administrative centre for the commotes or lordships of Meisgyn and Glynrhondda. The borough is first mentioned in 1262. The castle suffered capture and despoilment in the wars and riots of the closing years of the thirteenth century and earlier fourteenth century. It was repaired and continued to be maintained into the fifteenth century, but was ruinous by the early sixteenth century.

The castle consists of a small roughly oval court about 30m north-south by 25m. This had a stout wall with great round towers at the north and south ends. A single 13m tall fragment survives of the 14.2m diameter north tower, probably the 'Gilguran tower' of the early sixteenth century. Mere traces remain of the 10-11m diameter south tower. It is possible that there was at least one other tower. The ground falls steeply away on all sides but the north where there was a curving outer court, presumably also walled. In this area is the Guildhall (NPRN 31911) which may originally have been a medieval court house.

Source: RCAHMW Glamorgan III.1b 'The Later Castles' (2000), 185-195

John Wiles, RCAHMW, 12 February 2008