Three weeks of digging to excavate what could be the largest Roman garrison fort in Wales started on Monday.
The forts have been discovered at Dinefwr Park
The site, which dates from the first century AD, was first found at Dinefwr Park, near Llandeilo, in 2003.
Experts said the south Wales discovery could rewrite our understanding of the Roman conquest in the area.
Recent surveys confirmed the site, which is invisible from the surface, is much larger than first thought and is made up of two overlapping forts.
Emma Plunkett Dillon, archaeologist for the National Trust in Wales, said their teams would be digging nine trenches across the site.
"It is lifting the lid off selected areas of part of the site, to determine the character of what is buried beneath the soil because there is nothing to see on the surface," she said.
"We are all very excited about what we have discovered, and we look forward to taking the investigation a bit further.
"In work in 2003, we discovered a small fort, with sides about 100m in length - and we found just a small section - a tantalising hint - of a much bigger structure which appeared to be beneath this smallish fort."
Ms Plunkett Dillon said they think the larger fort, which underlies the smaller one, is around 3.9 hectares in size.
She said that, as well as the forts, the site also contained a civilian settlement, two roads, a possible bath house and "other enigmatic structures which we have yet to explain".
She added that the site could mean that the Roman conquest could be "a much more complicated story than has hitherto been understood".
"The discovery of this much larger structure really makes us all very excited, and makes us think we have discovered a different chapter, or a different interpretation into the invasion of south west Wales," she said.
Gwilym Hughes of Cambria Archaeology, which is undertaking the archaeological work in partnership with the National Trust, said the excavation was "a unique opportunity".
"Excavation will provide the critical dating evidence from items such as coins and pottery that may confirm when the forts were built and abandoned," he said.
There will be two open days at the site on 9 July and 16 July.