The origins of the timber Tregrug Castle in the form of motte and bailey date back to the 13th century, when the local lands passed into the hands of the de Clare family. The construction of a stone stronghold probably began at the end of the thirteenth or early fourteenth century, possibly on the initiative of Bogo de Clare. The work was probably not completed in 1314, when Gilbert de Clare died in battle with the Scots at Bannockburn. Then the castle was in the possession of the Despensers family, it was also attacked during the Welsh rebellion of Llywelyn Bren in 1316. After becoming the property of the Crown, it was sold in 1554 to the Williams family from Usk. During the English Civil War, the already ruined castle was again strengthened and garrisoned by the royalist Sir Trevor Williams. As a result, after the end of the war, Tregrug was demolished from the order of Parliament, because of fear of the military re-use by the royalists.
The castle consisted of a defensive wall, which surrounded a huge inner ward measuring 160×80 meters on a plan similar to a trapezoid. The entrance to its area was located through the south – west gate, consisting of two cylindrical flanking towers, with a wall thickness of up to 3 meters. From the inside it had two cylindrical communication towers. The second, smaller, but also twin-tower gatehouse was located in the curtain of the south-eastern wall. In the north-western part of the castle there was a rectangular keep with a side length of 10.4 meters with two towers in the eastern corners and an added semi-cylindrical tower from the west. In addition, the fortified walls were reinforced by four more round and semicircular towers.
Up to now, fragments of the eastern gatehouse have survived, of which the southern tower reaches 5 meters high, fragments of perimeter walls with heights from 1 to 5 meters and relics of the keep. The whole area is densely covered with vegetation and requires renovation works.
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