The castle was erected in the mid-13th century by Richard de Clare, Earl of Gloucester, lord of Glamorgan, to guard the newly conquered lands and the important route from the mountainous regions to the lowlands of Wales. Its construction probably took place in the years 1246 – 1252, after de Clare deprived the local ruler, Hywell ap Maredudd, although it could replace the older wood and earth fortifications from the 12th century.
In 1262, the castle passed into the hands of the lord Gilbert de Clare, who expanded it, but in 1295 it was devastated by the Welsh attack. The castle was damaged again in 1315 during the attack of the rebel Llewelyn Bren and in 1321 during the fights of the Marcher Lords against the hated favorite of the king, Hugh Despenser. In 1326, the unfortunate King Edward II was briefly imprisoned here before being taken to Berkeley Castle, where he was murdered.
Although the castle was still inhabited in the 16th century (a Raven Tower, probably a keep, was supposed to be inhabited, recorded by John Leland in 1538), it began to decline gradually. In the 19th century, the Marquess of Bute used Llantrisant stone for his extensive construction projects at the castles of Cardiff, Caerphilly and Castell Coch.
Built on a small hill, the castle consisted of two cylindrical towers, reinforcing from the north and south curved curtains of defensive walls, about 2.4 meters thick, separating a courtyard with dimensions of about 30 by 25 meters. The relics of the portal indicate that it was possible to get from the northern tower to the wall-walk crowning the walls. Its diameter was as much as 14.2 meters, while the southern tower was about 10-11 meters. The northern one had very massive walls, over 3 meters thick. Perhaps it served as a keep. Presumably, on its west side, since the end of the 13th century, there was a four-sided gatehouse, leading to the outer ward located north of the castle. Llantrisant might have resembled the better-preserved Castell Coch castle.
To this day, only a part of the northern tower has survived, reaching 13 meters in height and the remains of the south-west curtain. Only relics of foundations remained from the southern tower. Admission to its area is free.
Salter M., The castles of Gwent, Glamorgan & Gower, Malvern 2002.
Website coflein.gov.uk, Llantrisant Castle.