There seems to be two phases of ancient hillfort building on Carn Fadryn, followed by a mediæval fortification of the summit. The first period at Carn Fadryn at about 300 BC. Enclosed the summit and an area of some 12 acres (49,000 m2).
These were re-fortified during a second period, and enclosed a wider area towards the north of a total of about 26 acres (110,000 m2). This second period of reinforcement being at around 100 BC.
The third fort which strengthens the natural crag near the summit is thought to be “the castle of the sons of Owain”, mentioned in 1188 as being newly built. (Giraldi Cambrensis Opera (Rolls series, 1868), VI, p123 ‘dua castra lapidea de nova sita fuerunt; unum…Deutrait; alterum…in capite Lhein, quod erat filium Oenei, cui nomen Karnmadrun.’)