Dinefwr Castle Dinefwr Castle ( sometimes anglicized as Dynevor ) is a Welsh castle overlooking the River Tywi near the town of Llandeilo , Carmarthenshire , Wales . It lies on a ridge on the northern bank of the Tywi , with a steep drop of one hundred feet to the river . Dinefwr was the chief seat of the Principality of Deheubarth . The castle is owned by the Wildlife Trust of South and West Wales ( who do not charge for entry ) and is managed by Cadw but lies within Dinefwr Park , which is owned by the National Trust . Visitors who wish to see the castle and are driving there , may pay admission for the car park , if not National Trust or Wildlife Trust of South and West Wales members . A separate fee is payable if you wish to visit Newton House . Alternatively , access is free to walkers from Llandeilo town nearby , and there is a small free car park near the castle , but it can only be reached by a rough track through the deer park . Description . The present castle is entered via a massive door protected by a restored length of battlement . The short path from the car park reveals impressive views of the Towy valley . The door admits the visitor to the main space enclosed by the walls , from which there are several stairs to the main battlements and towers . A narrow spiral staircase leads to a high tower , which gives extensive views of the deer park to the north and the Tywi valley to the south and west . The castle keep is entered via the basementcellar at its base , but access to the circular walkway at the top can only be made via the battlement walk . Details such as the well and several garderobe s are visible in the structure . There is a path around the base of the main structure to the north . History . Traditionally , a castle was first constructed on this site by Rhodri the Great , but there are no remains from this period . Dinefwr later became the chief seat of Rhodri ' s grandson Hywel Dda , first ruler of Deheubarth and later king of the Britons , king of most of Wales . Rhys ap Gruffydd , ruler of Deheubarth from 1155 to 1197 is thought to have rebuilt the castle . Giraldus Cambrensis tells a story about a plan by King Henry II of England to assault the castle during a campaign against Rhys . One of Henry ' s most trusted followers was sent on a reconnaissance mission , guided by a local Welsh cleric . The cleric was asked to lead him to the castle by the easiest route , but instead took the most difficult route he could find , ending the performance by stopping to eat grass with the explanation that this was the diet of the local people in times of hardship . The planned attack was duly abandoned . Rhys ap Gruffydd also built the spectacular castle at Carreg Cennen , about four miles away to the south . It is not visible from Dynefwr , but Dryslwyn Castle can just be seen on a hill blocking the Tywi valley to the south - west . Rhys also founded two religious houses during this period . Talley Abbey was the first Premonstratensian abbey in Wales , while Llanllyr was a Cistercian nunnery , only the second nunnery to be founded in Wales and the first to prosper . On Rhys ap Gruffydd ' s death the castle passed to his son Rhys Gryg , and the earliest parts of the present castle are thought to derive from this period . Llywelyn the Great of Gwynedd was now extending his influence to this area , and Rhys , finding himself unable to resist , dismantled the castle . Llywelyn however had it restored and held it until his death in 1240 . In 1255 Llywelyn the Last gave Dinefwr to Rhys Fychan , then later gave it to Maredudd ap Rhys before later returning it to Rhys Fychan . Maredydd now allied himself to King Edward I of England , and helped Edward capture Dinefwr in 1277 . Maredudd had apparently been promised Dinefwr in return for his help , but Edward did not keep his promise and had Maredudd executed in 1291 . The castle now came into English hands , though it is recorded to have been burnt during the rebellion of Llywelyn Bren in 1316 . In 1317 it was given to Hugh Despenser , the king ' s favourite . It was unsuccessfully besieged by the forces of Owain Glyndŵr in 1403 . Towards the end of the 15th century the castle was held by Sir Rhys ap Thomas , who carried out extensive rebuilding . In 1531 his grandson was executed for treason and the castle was confiscated by the crown , though the family were later able to recover it . Around 1600 Newton House was built nearby and the castle keep modified as a summer house . The remains of the large windows can be seen at the top of the keep , but it burned down in the 18th century . The castle is now owned by the Wildlife Trust of South and West Wales and managed by CADW on their behalf .