The ruins of the medieval timber castle can be found on a hill about 1½ miles up the valley from the seaside town of Aberarth. The founder is thought to be one Richard de la Mare, a follower of Richard Fitz Gilbert, Lord of Clare, an Anglo-Norman lord, who, by the archaeological evidence, built it on top of a previous defensive position.
The castle was razed by Gruffydd ap Rhys but probably rebuilt, as it is known to have been destroyed again by Owain Gwynedd in 1136. The castle probably spent the next fifty years passing from one feudal ruler to another: from Hywel to Cadwaladr in 1144, and then ceded to Roger de Clare, 3rd Earl of Hertford in 1158, who garrisoned it. Destroyed by The Lord Rhys in 1164, it came into the possession of Maelgwn ap Rhys who lost it to, and recovered it from, his brother, Gruffudd ap Rhys . Maelgwn dismantled it to prevent his lands being held by Llywelyn the Great of Gwynedd. As Llywelyn then captured and held all the territory from the River Arth to the River Aeron, he conferred Dinerth on the sons of Maelgwn's brother Gruffudd as was rightfully due. The site was abandoned soon afterwards.
The supposed pre-Norman use of the site as a defensive position has led some historians to conjecture that the site may have some connection to the legendary King Arthur and Camelot. However although din means 'fort' in old Welsh and arth means 'bear', they are both common placename elements in Welsh and the name of the fortress is logically more likely to derive from the River Arth which flows close by rather than to mean 'Fort of the Bear'. The castle has also been called "Hero" castle  (in Norse hiro means 'king's carl').
A receptaculi ursi is referred to by King Arthur's contemporary, Gildas. This means 'lair or retreat of the bear' and is sometimes translated as 'Fort of the Bear'. However, it is more likely to refer to a different Dinerth - a sub-Roman hillfort on Bryn Euryn above Llandrillo-yn-Rhos in Conwy county - because it is mentioned in connection with King Cynlas of Rhos.
- "Castell Dinerth". Retrieved 12 January 2009.
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