Morlais Castle is situated on the summit of a naturally defendable limestone escarpment overlooking the Taf Fechan. Its location, paired with the riverine resources, saw it used as a fortified settlement during the Iron Age but little is currently known about this period of occupation.

Morlais was situated in the extreme northern limit of the Lordship of Glamorgan which came under the control of the de Clare family in 1217. The remote location, far removed from the main lines of movement through Glamorgan, meant the site was of little interest to this powerful Norman family. In 1262 the Lordship passed to Gilbert de Clare, Earl of Gloucester who initially supported Simon de Montfort in his rebellion against Henry III but later changed sides supporting Prince Edward (later Edward I) in his campaign that culminated in the Royalist victory at theBattle of Evesham (1265). Henry III granted him Abergavenny Castle but Gilbert focused his efforts on a much more elaborate structure; in 1268 he commenced building Caerphilly Castle as a vast statement of power and a provocative challenge to the native Welsh.

Gilbert’s power peaked in 1282 when he led an unsuccessful expedition into Southern Wales as part of the Second War of Welsh Independence. Defeated at the Battle of Llandeilo Fawr, he was relieved of command undermining his position as a Marcher Lord. In an attempt to re-establish his power, he commenced construction of Morlais Castle circa-1288. Built on the northernmost limits of the Lordship of Glamorgan overlooking neighbouring Brecknockshire, it was a particular grand structure occupying the summit of a hill. A triangular Inner Bailey was dominated by a large Round Keep whilst access to the Outer Bailey was guarded by a large 'D' shaped tower.  Unusually the Outer Ward also had a large Keep-like Round Tower - the rib-vaulted basement still survives - whilst the Great Hall, or some other substantial building, seems to have been built along the inside length of the curtain wall. A cistern, a deep pit for collecting rainwater, was located in the centre of the enclosure.

The building of the castle immediately ignited tensions with Humphrey de Bohun, Earl of Hereford and Lord of Brecknock who claimed it impacted on his sphere of influence. The dispute escalated culminating in a skirmish known as the Battle of Maes Vaynor (1291). In an unwelcome distraction from his other military ambitions, Edward I intervened to settle this local but potentially destabilising civil war. Both Gilbert and Humphrey were fined and spent a short period incarcerated in the Tower of London.

Due to his role as the aggressor, Gilbert had been fined the greater sum – 10,000 marks – on account of his building of Morbais Castle and it is not clear work on the castle ever resumed although it certainly remained in use as the castle was captured by the Welsh during the revolt of Madog ap Llywelyn in 1294. Although this had started in North Wales, it had ignited a wider rebellion and a number of castles, including Morlais, were captured at this time. The Welsh in Glamorgan made peace with Edward I in 1295 and Morlais was eventually returned to Gilbert de Clare.

The death of Gilbert, in December 1295, saw Morlais abandoned. The castle drifted into ruin but its walls were still standing to a significant height in 1741 for they were captured in an engraving by the Buck brothers. Thereafter the stonework was robbed to support local building endeavours.

ref:  http://www.castlesfortsbattles.co.uk/morlais_castle.html