A previously unknown Roman fort discovered in Pembrokeshire in Wales overturns assumptions that the area’s indigenous Celtic tribe was on peaceful terms with the Roman invaders.

The site, which has excited archaeologists, had been hidden until now beneath an enormous, overgrown field. It explains why the land had been unsuccessful for farming: the farmer kept hitting stone.

The discovery was made by Dr Mark Merrony, a leading Roman specialist and tutor at Oxford University, who said: “It is a humongous fort, an incredible find of national importance.”

He is all the more excited because it is right next to a Roman road that he has also identified for the first time.

The fort is thought to date from the first to the third centuries, when the Celtic Demetae tribe inhabited the south-west area of modern Wales.

They were thought to have been pro-Roman, meaning there was less need for a major military presence to quell local resistance.

Merrony said that this fort suggested this part of Wales was considerably more militarised than previously thought: “I now don’t think they were pro-Roman at all, but that the Romans were hitting the area with an iron fist.”

He noted that its form and scale was like the only other Roman fort known in Pembrokeshire, excavated at Wiston near Haverfordwest in 2013. Both forts were now linked to a Roman road network that had not previously been known, he said.