Morlais Castle has quite a spectacular setting atop an isolated limestone hill above the Taf Fechan valley, 2 miles north of Merthyr Tydfil, looking across to the high peaks of the Brecon Beacons and many lesser hills. Actual remains, however, are very limited, in part due to the exposed location which meant that once defensive requirements lapsed, the castle site was not suitable as a residence and hence it was abandoned at an early stage, probably just a decade after its construction, following damage sustained during Welsh insurgencies.
Although positions of walls and towers are clearly evident from aerial photographs, on the ground the castle is marked only by low mounds, shallow ditches and formless piles of stones, with only four easily recognisable structures; a moat to the south and east, a deep cistern at the centre, a projecting wall fragment along the east side and, by far the best part, a vaulted basement entered via steps through a strongly arched doorway. This enclosure formed the lower level of the largest tower, the south keep, and although the flat roof has been restored, the underground chamber is unchanged since construction in the late 13th century. It is quite striking to descend from the featureless mounds and stone piles on the often wind-swept hill, into the dark, protected confines of the cellar, supported by sturdy medieval vaulting. The site is also notable for cliffs, terraces and ledges that line the hill to the north and west, remnants of a limestone quarry that operated here in the 19th century.
The castle is sited at the northwest corner of Morlais Hill, a limestone plateau that slopes gradually down to the south and east but is lined by sheer cliffs on the other sides, present even before the establishment of the quarry. Most of the remainder of the hilltop is now used as a golf course. Access to the castle, and the old quarry, is from a narrow road between Merthyr Tydfil and Pontsarn, parking on the west side just south of the bridge over the Taf Fechan river; the castle is a third of a mile east, along paths through woodland then up a grassy slope between two sections of the quarry, ascending nearly 400 feet.
The main section of the castle, the inner court, occupied the northern third of the site - a triangular enclosure with a large circular tower at the apex (the north keep) and a smaller tower to the southeast; the southwest corner had no tower. Both towers are evident only as circular hollows. Within the court were several stone buildings including a kitchen and a hall. Walls continued south around the larger outer court, incorporating four more towers, of which only one is easily discernible - this is the south keep, the ground floor of which has been restored and is now a neat circle of flat, mortared stones. Below here a flight of eight steps leads down to the vaulted, Gothic-style basement chamber which is approximately circular in outline, 9 metres in diameter, the ceiling supported by a central column and 12 radiating ribs, linking with vertical pilasters down the walls, joined by blind arches. The south keep tower is linked with a smaller tower to the south via the best-surviving section of curtain wall, though even this barely rises above the surrounding rubble. The outer court also contained several buildings, along the west and east sides, their walls now marked by faint grassy banks, while near the centre is the cistern, a vertical-sided hole, square in cross-section, intended to store rainwater, currently 11 metres deep but believed originally to have been twice this. The one entrance to the castle was along the east wall, guarded by a two floor gatehouse; the projecting wall fragment is part of this structure. The south and east edges of the fortified section of the castle are protected by a ditch, or moat, cut into the limestone bedrock, and still up to 5 metres deep in places; as with the cistern the original depth is masked by debris. South of here are two lesser protected areas, the inner and outer baileys.
Morlais Castle was constructed by Gilbert de Clare, 3rd Lord of Glamorgan, beginning around 1288, and was located along the northern edge of his territories, though ownership of the site was disputed by Humphrey de Bohun, 3rd Earl of Hereford, who occupied adjacent Brecon to the north. The castle was attacked, damaged and captured in 1294, by Welsh insurgent Madog ap Llywelyn, possibly while some sections were still incomplete, though English control was soon restored, and the ruins were visited by Edward I in June 1295. Despite the substantial effort and expense that had been expended in order to construct this large castle in such a difficult location, it seems that the fort was never repaired or subsequently occupied, and so it began a long process of decay. Nonetheless as late as 1741 (as shown in a contemporary engraving), walls of the north keep tower remained up to second floor level, yet these had completely gone within the next century. The site now is not obviously managed or even signposted.