Craig Gwrtheyrn occupies a commanding bluff about 1.2 miles west of the village of Llanfihangel-ar-Arth in Carmarthenshire. The fort itself follows the contours of the hill, enclosing about 1.5 hectares with a defensive line measuring roughly 160 metres north to south and 125 metres east to west. The original drystone rampart was approximately 2 metres thick. A surrounding ditch reinforces the line everywhere except the very steep north-east face, where the natural drop made fortification unnecessary. Stones from the collapsed rampart now litter the ditch. The single entrance is at the south-west, where two further banks form a barbican, a defensive elaboration designed to channel attackers into a killing zone. And there in the entrance area, set on end and still partly visible, are the spiked stones.

Chevaux de frise is a French term, literally Frisian horses, originally referring to a portable timber and spike barrier developed in the 17th century to stop cavalry. Archaeologists borrowed the name for the much older stone-spike defences found at a handful of British and Irish Iron Age sites. The most famous British examples are at Castell Henllys in Pembrokeshire and at the broch sites of northern Scotland; the Spanish hillforts of Iberia also have spectacular examples. They are remarkably rare in Wales, and Craig Gwrtheyrn has one. The point of such a device is to break formation and slow the approach so the defenders can deal with attackers piecemeal from the ramparts. The thirty surviving stones suggest there were once more, lost to ploughing or robbed for walls in the surrounding fields, but enough remain to mark out the original line.

The hillfort's modern name attaches it to one of the most argued-about figures of post-Roman Britain. Vortigern, in Welsh Gwrtheyrn, was a 5th-century British warlord whose name appears in several early medieval sources, including Gildas's 6th-century De Excidio Britanniae and the 9th-century Historia Brittonum compiled by an unknown writer often called Nennius. In these accounts, Vortigern is the king who invited the Saxons Hengist and Horsa into Britain as mercenaries against the Picts and Scots, only to find them turning on him and seizing land. Bede repeated the story in the 8th century. He may have been a real person; he may be a composite, or even a title (gwrth-tigern means something like overlord in Welsh). Either way, by the medieval period he had become the embodiment of British failure: the king who let the enemy in.

According to one of the accounts in Historia Brittonum, Vortigern fled persecution by St Germanus of Auxerre, a 5th-century Gaulish bishop who came to Britain to combat the Pelagian heresy. The king, the chronicle claims, built a stronghold on the River Teifi and took refuge there with his family. Germanus pursued him. Fire descended from heaven in answer to the saint's prayers, and the castle, the king, his family, and all his retainers were consumed. Other places along the Welsh coast also claim Vortigern's death; Nant Gwrtheyrn on the Llyn peninsula has a rival tradition. Whether the hillfort acquired its name because of the legend, or whether an earlier folk memory of the place attached the legend to it, no one can now say. The story sits in the bracken alongside the chevaux de frise, two kinds of evidence about the same hill, each as silent as the other.

Climb the southern bank of Craig Gwrtheyrn on a clear day and the Teifi valley spreads out below: the river winding eastward toward Llandysul and Newcastle Emlyn, the patchwork of pasture and woodland on both sides, the long Cambrian foothills rising blue in the distance. The fort is a scheduled monument and lies on a public footpath, though access is via narrow lanes and the surrounding land is in active farming. It has not been excavated in modern times, and what archaeology might find under the turf, beyond the spikes and the bank, is unknown. The name endures. Local people still know it as Vortigern's hill. The 5th-century warlord, if he existed, almost certainly never stood on this particular bluff. But his name is on it, and the wind passes over it the same way the wind passed over it a thousand years ago when somebody first thought to put him here.