Kidwelly Castle began as a semi-circular earthwork built on a ridge above the River Gwendraeth by Roger de Caen, Bishop of Salisbury, early in the 12th century. A memorial outside the main gate of the castle commemorates the death of Gwenllian - wife of Gruffudd ap Rhys, Lord of Deheubarth - who was killed in battle as she led Welsh forces against the castle in 1136. The victor that day was Maurice de Londres, and his heirs kept the castle until 1216, although they had to endure several successful Welsh raids during this period.
By the early 13th century a stone curtain wall had replaced the timber palisades around the earthwork, but it wasn't until the late 13th century, when the castle was owned by the Chaworth family, that a major reconstruction in stone took place. A square enclosure with round corner towers was built in the courtyard. The inner ward was entered through two weakly defended arches in the curtain wall, suggesting that a strong gatehouse must have existed in the outer defences. The outer curtain wall was rebuilt with four mural towers and a small gatehouse was added to the north. The height of the inner defences was greater than that of the outer defences allowing archers on the inner walls and towers to fire over the defenders on the outer walls increasing the firepower aimed at any attacking force. This arrangement of concentric defences is similar to those at other great castles built in Wales at the time, including Edward I's castles at Harlech and Beaumaris and Gilbert de Clare's castle at Caerphilly.
Construction continued at the castle with the outer curtain wall raised and three of the inner ward towers heightened to maintain their line-of-sight over the outer wall. New domestic buildings were built and a chapel tower was added that projected out towards the river.
By the mid 14th century the castle had become an important administrative centre for the Duchy of Lancaster. In 1389, John of Gaunt, Duke of Lancaster, began construction of a large new gatehouse at the southern end of the outer ward. By 1403 it was strong enough to withstand an attack by Owain Glyn Dwr, but subsequent damage by fire and changes to the original plan meant that it wasn't completed until 1422. Further buildings were added to the outer ward in the late 15th century, but by the early 17th century the castle was already falling into ruin.