The first historical record of Dinefwr Castle dates back to the rule of Rhys ap Gruffydd, a Welsh prince of the province of Deheubarth, noted for his victories against the encroaching early Norman rulers of England. A fortification may have existed at the site prior to that time, but it is believed that the first stone castle was probably built sometime between 1155AD and 1197AD, during Rhys's rule.

Following the death of Rhys ap Gruffydd in 1197AD, his sons fought over the succession, each claiming a right to the kingdom of Deheubarth. Llywelyn the Great of Gwynedd took this opportunity to push into the south of Wales, and in 1216AD, he enforced a division of the kingdom of Deheubarth between the three brothers. Dinefwr Castle passed to Rhys Gryg, but with none of the status it had previously enjoyed as the centre of a powerful province. Regardless of its changed circumstance, it is thought that Rhys sought to improve the castle, and the earliest surviving remains of the current ruin are believed to date to this period.

When Rhys Gryg died in 1233AD, his territory was divided between his sons, and Dinefwr Castle passed to Rhys Mechyll. Rhys Mechyll died in 1244AD, and was succeeded by his son, Rhys Fychan - a supporter of the English king. In 1256AD, Llywelyn the Last invaded the demesne of Dinefwr Castle, removed Rhys Fechan, and granted the lands to his uncle, Maredudd. Eventually, Llywelyn the Last returned the castle to Rhys Fychan, with whom it remained until his death in 1271AD. At this time, it passed to Rhys Fychan's son, Rhys Wedrod.

In 1267AD, Henry III formerly recognised the title "Prince of Wales", and peace existed between England and its neighbour. However, following Henry's death in 1272AD, Edward I ("Longshanks") succeeded to the throne of England. In 1276AD, Edward declared Llywelyn a rebel, and an English army was dispatched to break the power of the Welsh prince. LLywelyn sued for peace, and Dinefwr Castle was placed into English hands. From then on, it remained largely in the possession of the English.

The crown placed Dinefwr Castle into the care of a constable, and between the years 1282AD-1283AD, the ditches were cleared, the tower, bridge and hall were repaired, a new gate was installed, and further buildings were erected within the outer ward.

In 1287AD, Rhys ap Maredud rebelled against the English, capturing the castles of Dinefwr, Carreg Cennen and Llandovery. The English responded by raising an army 11,000 strong, and swiftly recaptured all lost territory, and subsequently executed Rhys for treason. In 1298AD, "New Towne" was established near the castle, and settled with English families to counter the growing Welsh population growing up around the Dinefwr Castle.

In 1316AD, Dinefwr Castle was torched during the rebellion of Llywelyn Bren. In 1317AD, the castle was granted to Hugh Despenser - a favourite of the king, but unpopular amongst the Marcher Lords of the Welsh border. The latter rose up against him, attacking his Welsh holdings, including Dinefwr Castle. Extensive repairs were carried out at the castle following this incident. Following this, however, little was undertaken in the way of maintenance at Dinefwr, and by 1343AD, the great tower was recorded as being near a state of collapse.

In 1403AD, the castle was unsuccessfully besieged by Owain Glyndŵr, and further repair work was undertaken. The castle enjoyed a period of relative quiet following this incident, but also one of decline. Whilst the surrounding district prospered, the need for a medieval fortification decreased, and Dinefwr Castle was eventually abandoned in favour of the first incarnation of the nearby Newton House.

In 1600AD, the present Newton House was founded, and Dinefwr Castle was brought into service as a summer house - accounting for the unusual round extension on top of the keep. This survived until the 18th Century, when the summer house was destroyed by fire.

Today, Dinefwr Castle is owned by the Wildlife Trust of South and West Wales, and managed by CADW. It sits in the grounds of Newton House, and although entrance to the castle is free, Newton House impose a charge for parking. It is approached via a modest uphill walk, through pleasant fields, woodland and a deer park.

Much of the castle survives, including the curtain wall, towers and modified keep. Walks around the battlements provide spectacular views out across the surrounding countryside. Although Dinefwr isn't one of the most impressive Welsh castles, the overall ambience of the location and the ruin make a visit worth the effort.