Also called Castell Meredydd, fragments of a keep and curtain footings are all that are left of the only native Welsh castle in Gwent. It was built in the late 12th or early 13th century on a clifftop site.
The castle consisted of a round tower on a mound, inside a rectangular enclosure. A second mound stands beside the castle motte, and may have been topped with a residential building. The mounds are roughly 4-7 metres high with a ditch defense on one side. The round tower was 10 metres in diameter with walls nearly 2 metres thick. The rectangular building atop the second mound originally measured about 18 metres long and 10 metres wide.

The castle is generally attributed to Maredudd Gethin, prince of Gwynllg, though there are alternate theories. It probably was abandoned shortly after 1236. It was thought to be used by Morgan ap Hywell after he was ousted from Caerleon by the Normans, and it is possible that Morgan built the round tower. The curtain wall and enclosing bailey were probably later additions, possibly built by Gilbert Marshall, the Norman Earl of Pembroke, after he captured Castle Machen around 1236. The castle was held by Maredudd after 1248, and later passed to the powerful de Clare family.

While in the area, take the time to visit Caerphilly Castle, only 4 miles away.