Update on the Conquest of Britain
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.