Kidwelly Castle is an imposing ruin, situated on a scarp above the upper tidal limit of the Gwendraeth Fach Estuary. It was built for Roger, Bishop of Salisbury (1102-1139) as the centre of a significant lordship comprising the commote of Cydweli. The castle was established between 1106 and 1115, at the same time as the Benedictine Priory, now St Mary's Church (NPRN 301847), located some 180m south (upstream, on the other opposite side of the estuary). The priory was also contemporary with Capel Cadog (NPRN 418), some 360m to its north-north-east. By the 1130s the castle was held by Maurice de Londres, Lord of Ogmore, Glamorgan. The castle was held by the familiy until 1216, although there were intervening periods when the castle was in Welsh hands, following its capture. In the 1160s the castle was held by the Lord Rhys (Rhys ap Gruffudd), and it is thought that the earliest survivig piece of masonry in the outer curtain wall may date to this time. In the early thirteenth century the castle passed to the Chaworth family. In 1283 the king granted Kidwelly to William de Valence, Earl of Pembroke (died 1296), but on his death it reverted to the Chaworth heiress, Matilda (who married Henry Earl of Lancaster in 1345). The castle passed to John of Gaunt, Earl of Lancater in 1362.

The castle is a Grade I listed building, considered one of the finest castles in Wales. Roger's castle consisted of a D shaped ringwork (the line of which dictated the position of the later outer curtain wall), constructed of earth and timber. The south-west section held the small medieval walled borough (NPRN 33065). The castle gardens (NPRN 79038) occupied its north-east section, which, with the castle itself, is a Scheduled Ancient Monument. No trace remains of the original castle buildings. The castle's square inner ward was the first phase of building work carried out by the Chaworth family. It has a circular tower at each corner and is thought to have been completed by 1283, when the king stayed for several days. The existing hall and chapel were built slightly later than the original internal buildings, about which little is known. The castle's outer ward, constructed around 1270-1290, consists of a D shaped enclosure with north and south gates and three D shaped towers. It is thought that William de Valance continued building work in the late thirteenth century. The outer curtain wall was rebuilt at around 1300, with only part of the central section retained. The remains of this run from the tower closest to the great gatehouse to the north gate and the adjacent (collapsed) tower, where it meets the inner ward's north tower. In the late fourteenth century a new hall with solar was constructed along the inner ward's east side, with a new kitchen opposite. The chapel was built around this time, in a tower projecting down the scarp below the south-east tower. The outer ward's towers and curtain walls had their tops raised at this time. The inner ward's north-east and both west towers were heightened.

It seems likely that the court walls were never visible from outside the castle, being overshaddowed by the outer wall. This would have effected the visual and symbolic impact of the four great towers, each one sufficient to signal lordship at a lesser castle. The castle was maintained into the sixteenth century.

Sources include:
Cadw, Guide to Kidwelly Castle, revised 4th edition (2007)

John Wiles, RCAHMW, 26 Janiuary 2008