Llantrisant Castle, once a proud edifice in medieval times, lies at the central core of the old hilltop town and represents a magnificent towered medieval fortress now reduced to fragments.
Its remains of a partial tower and small bailey make it the only medieval monument in the county borough and these are protected by Cadw's Scheduled Ancient Monument listing. The castle is a small Glamorgan courtyard castle with its commanding panoramic views of the Vale of Glamorgan and the north Devon coastline.
Tradition that Robert Fitzroy, the first Earl of Gloucester, Lord of Glamorgan (died 1147) built the castle is unproven, but a Norman church built from around 1096 onwards supports claims for an early foundation of the castle. The timber defence with ditches could have been developed as a stronghold.
We can be certain that Llantrisant Castle was fortified in 1246 on the orders of Richard de Clare, Earl of Gloucester and Lord of Glamorgan as the administrative centre of the lordships of Meisgyn and Glynrhondda in the kingdom of Morgannwg and has stood as a landmark to the Norman French influence in South Wales for almost eight hundred years.
At the height of its power Llantrisant was rated as 'second only to Cardiff in military importance' as De Clare needed an administrative centre for annexed Welsh lands in the hill district of Meisgyn and where better than the elevated and defensible position of Llantrisant.
He had secured possession of his estates in 1243 and began to consolidate his rule in Glamorgan. The boundary of effective administration was extended northwards into the hills and valleys when Hywel ap Maredudd, the Welsh lord of Meisgyn, was defeated in 1244-5 and his lands swallowed by de Clare’s bureaucracy.
A record of 1246 clearly states that Earl Richard “fortified the castle of Llantrisant (Castrum de Lantrissen) having driven out Hywel ap Maredudd” which doesn’t exclude the possibility that this building involved a refortification of an earlier castle. The stone castle is likely to have been built on the site of an earlier timber fortification erected by Lord Gwrgan ap Ithel and repaired, if not rebuilt by Einion ab Collwyn after the invasion of Fitzhamon.
The castle is sited on a projecting spur of pennant sandstone from the steep southerm slope of the Llantrisant hillside. When the Gigfran (Raven) Tower was intact, these views stretched across the conquered south and the perilous north or Blaenau.
Its strategic position, guarding the important route from the upland to the lowland, is very apparent. It probably had two towers, with the second to the south which adjoined a curving out court, presumably also walled and in this area the Guildhall – a medieval court house – stands.
In 1252 Margaret, De Clare’s fifth child, was born in the castle while his eldest son, Gibert “The Red” went on to build Caerphilly Castle in 1268. Certainly by 1262 the small borough of Llantrisant was developing rapidly beside the castle walls although it suffered capture and despoilment in the wars and riots of the closing years of the 13th century and earlier 14th century.