The fort was found at Dinefwr Park

The discovery on National Trust land at Dinefwr Park near Llandeilo could greatly improve understanding of the history of the area.

Special mapping techniques were used to identify the fort below the surface, although its exact location is being kept a secret to allow for further research.

One day it is hoped the site could become a flurry of activity as the hidden community is uncovered.

Cambria Archaeology was commissioned by the National Trust to undertake surveys of the parkland.

Hopefully we will carry out some further archaeological investigations Gwilym Hughes

Emma Plunkett Dillon, an archaeologist with the National Trust, said there had been suspicions for some time of Roman activity in the area.

"Hints over many years had suggested that there was Roman activity in the are," she explained.

"We have been able to take advantage of archaeological techniques which allow us to discover what is beneath the surface without actually having to disturb the ground in anyway.

"We ended up with such a clear picture we could identify the ramparts and the street layout."

At Dinefwr a square or rectangular fort, only part of which was detected up by the survey, is overlain by a smaller rectangular fort apparently packed with buildings and streets.

The fort is within a day's march of Carmarthen and Llandovery

Both follow the expected pattern for a Roman fort, a layout replicated throughout the empire.

It is potentially one of the largest campaigning forts in Wales and may transform understanding of the Roman conquest.

Gwilym Hughes, director of Cambria Archaeology, said it was an exciting find.

"It is an exceptional fort," he said.

"We know that there are Roman forts in Carmarthen and Llandovery and always suspected that there was a fort here in Llandeilo because it is half way between these two towns and within a day's march of the two sites.

"Essentially it looks to me as though there are two forts - the early one dating to the immediate period of the Roman conquest of south west Wales - probably 70AD to 74AD.

"That appears to have been abandon soon after the conquests were successful.

"Then they appear to have returned and established a second fort over the top of the original fort, perhaps later that same decade.

"Hopefully we will carry out some further archaeological investigations."