The castle at Llanblethian, near Cowbridge in Vale of Glamorgan, is relatively small, and its outer defences were probably never completed, though it has an impressive setting, on a wooded ridge protected on three sides by the River Thaw, and retains one main feature, an imposing, twin-towered gatehouse, its walls still mostly complete to second floor level. Also known as St Quintin's Castle after the de St Quintin family who oversaw the initial phase of its construction, the first fortification was installed at the start of the 12th century - a timber structure surrounded by a circular bank and ditch, soon replaced by a stone keep, the low overgrown remains of which can be seen at the centre of the site.
The main building work happened at the start of the 14th century, after ownership had passed to the de Clare family; the land around the keep was enclosed by low walls in a quadrangular arrangement, protected along the vulnerable east side by two corner towers and the gatehouse. The last inhabitants of the castle left around the start of the 18th century and the place quickly became ruined, being used at one point as a shelter for cows. The towers are now merely foundations and most of the curtain walls are just a few feet tall, but the gatehouse retains many original features and its first floor level is still accessible.
Llanblethian Castle is located just south of Cowbridge on the A48, where the River Thaw loops around three sides of a low limestone ridge, and the place is reached by Castle Hill, a narrow, quiet residential street. The site is managed by Cadw, is free to enter, open at all times, and has a small parking area at the northeast side, next to the remains of one of the two towers that helped defend the eastern frontage. This tower has a rectangular outline and the surviving walls are up to 2.5 metres high, mostly just the internal coursing, lacking the more ornate facing stones. It is thought that the tower originally rose to the same height as the three-storey gatehouse, and held living quarters within its upper two levels. The second main tower was to the southeast - this has a polygonal outline and is even less complete. The two towers were connected to the gatehouse by short but thick curtain walls, and all three of these eastern structures are unusual in that they project fully out from the inner bailey, rather than being partly or wholly built within the walls.
The other three sides of the bailey are ringed by lower walls, between one and two metres high, this close to their original height, as the steep slopes beyond are presumed to have offered sufficient protection. At the centre of the north wall is a wide gap, perhaps the site of a gate, but more likely just a minor entrance in use during the construction phase. The only other feature around the perimeter is a small turret at the northwest corner. Approximately at the centre of the site is an overgrown mound topped by several large trees, with the fragmented remains of the original keep; both the early masonry from the start of 11th century, and a few blocks of later, better quality stonework indicating that the keep was retained and enlarged once the outer defences were added in the early 1300s.
The main component of St Quintin's Castle is the large gatehouse, well constructed out of grey limestone; this consists of a central passageway between two equally sized towers, the inner sides flush with the curtain walls that link to the corner towers. The front half of each tower is chamfered at the corners, creating a half octagonal frontage. Pairs of grooves around both ends of the entrance passage show that it was guarded by two portcullises in addition to the wooden doors, and the entrance was further protected by arrowslits in the vaulted rooms (guard chambers) on the ground floor level of the flanking towers. Originally the entrance door was positioned above a steep-sided ditch, but this has long since been filled in. One of the guardrooms is usually open while the other is locked, seemingly used as a store for maintenance equipment. Some of the arrowslits in the guard chambers have been modified, and converted to regular windows, modifications done in the 15th century when the castle was transformed into a prison. Steps on the north side of the entrance passage lead up to the first floor, where railings demarcate a short pathway, bridging over the roofless entrance passage, across to a closed doorway that led to the southern wall walk. Other features on the upper floor include windows, arrowslits and a shallow fireplace, plus beam supports for the floor of the uppermost, second floor level, the walls from which are mostly missing.