The fort was discovered by Dr Mark Merrony, a Roman specialist at the University of Oxford, who heads an excavation of the site about 3.5km south-west of the summit of Frenni Fawr, the easternmost peak of the Preseli Hills. Satellite imagery had shown a convincing outline in an overgrown field, and digging has since uncovered fourth-century coins among other finds.
The discovery breathes life into "The Dream of Macsen Wledig", one of the medieval Mabinogion tales, which describes how Magnus Maximus, on military manoeuvres, pitched his camp at Frenni Fawr, a 395-metre peak. The emperor ruled Britain in the fourth century and led troops against the Picts and Scots. Until now, no archaeological evidence of a Roman marching camp or fort had been found on or below the summit.
Merrony said he was excited that the landscape held the only Roman military site close to where the Mabinogion places the emperor. "If Magnus Maximus was there, there must have been an army there," he said. He added that the presence of a heavily fortified Roman auxiliary fort may help unlock the mystery of a legend that has endured for nearly 1,700 years.