The earliest defenders arrived around 300 BC. They enclosed the summit and roughly twelve acres of upper slope behind a stone rampart whose footings can still be traced through the heather. Around 100 BC their successors decided the lines were not far enough out and built again, this time enclosing some twenty-six acres reaching toward the north. The collective area inside these two walls makes Carn Fadryn one of the larger Iron Age hillforts in north-west Wales, a five-hectare site whose population in any given generation might have run to several hundred souls, with cattle and grain stores stockaded inside the upper enclosure during the worst weather and the worst raids.

More than a thousand years after the Iron Age builders, a third fortress went up here. Gerald of Wales, riding past in 1188, took the time to record it in his Itinerarium Cambriae: dua castra lapidea de nova sita fuerunt; unum...Deutrait; alterum...in capite Lhein, quod erat filium Oenei, cui nomen Karnmadrun. Two stone castles newly built, one of them at the head of Llyn, belonging to the sons of Owain, called Carn Fadryn. Owain Gwynedd, the great twelfth-century prince of Gwynedd, had died in 1170, and his sons divided his territory between them. Carn Fadryn's medieval fortress, perched on the natural crag near the summit, was the visible symbol of one son's portion. It was among the earliest stone castles built by Welsh hands, anticipating the great Edwardian fortresses of Caernarfon and Conwy by a full century.

The summit today is grass and crag and the tumbled stones of three different ages of wall, all of them softened by lichen and weather and indistinguishable to anyone but an archaeologist. Sheep crop the grass between the ramparts. The footpath up from the village of Garnfadryn climbs steadily for an hour or so, and the final scramble to the cairn at the top can be windy enough to lean against. On the right day every horizon delivers, the kind of view that puts the rest of the Llyn into its proper geographical place. On the wrong day, the cloud closes in below the summit and you walk the ramparts in a grey hush, feeling rather than seeing the four counties around you and understanding why the Iron Age people decided this hill was the place to make a stand.