Craig Gwrtheyrn, or Caer Gwrtheyrn, is an Iron Age hillfort on the south side of the River Teifi, about 1.2 miles (1.9 km) west of the village of Llanfihangel-ar-Arth, in Carmarthenshire, Wales.

The defences measure about 160 metres (520 ft) north to south, and 125 metres (410 ft) east to west, following the contours of the hill, enclosing an area of about 1.5 hectares (3.7 acres).

The drystone rampart, of which traces remain, was originally 2–3 metres (6.6–9.8 ft) thick.

There is an entrance on the south-west, where two further banks create a barbican and where there is also a small area of chevaux de frise: sharp stones set on end, of which about 30 remain.

According to one story (in Historia Brittonum, compiled in the 9th century) Vortigern, fleeing from St Germanus, built a castle on the River Teifi.