Dryslwyn Castle is built on the same ridge as Dinefwr Castle. It is likely that Lord Rhys, the ruler of Deheubarth in the 12th century, maintained a stronghold in both places, although both castles were rebuilt in stone by later rulers.
Dryslwyn Castle as it exists today “stands on top of a hill overlooking the Tywi valley. Its date of construction is unknown but the similarity between it and neighbouring Dinefwr Castle suggest that it was built at a similar time and possibly by the same person. The most likely builder was Rhys Gryg who occupied Dinefwr in the early 13th century, or possibly his son Maredudd, who inherited Dryslwyn from his father.
By the late 13th century the castle at Dryslwyn had developed into the largest native Welsh castle in South Wales. In 1277 the English king, Edward I sent an army into Wales to defeat Llywelyn ap Gruffudd. Maredudd’s son, Rhys, who had inherited Dryslwyn after his fathers death in 1271, surrendered without a fight and was allowed to keep his castle. Dinefwr Castle was not as quick to surrender and as a result was forfeited by the king. Dryslwyn now had an English neighbour, a situation that was not well received by Rhys who felt he had a claim to the lands. In 1287 Rhys, enraged by years of border disputes with his English neighbours, captured the castles of Dinefwr,Carreg Cennen and Llandovery. The English response was swift and an army of 11,000 men recaptured the castles and defeated Rhys after a three week siege at Dryslwyn. Rhys escaped but was eventually captured and executed for treason.” http://www.castlexplorer.co.uk/wales/dryslwyn/dryslwyn.php
Can anyone say irony. During the 1282 war, Rhys found allegiance to Edward better suited his needs and did not support Llywelyn ap Gruffydd in his quest to maintain and independent Wales. Whoops.