Archaeologists said the original discovery was to rewrite Exeter’s early history.
Now the excavations have revealed what the experts believe could be a second fort, built on top of the first.
Tim Gent, head of Exeter Archaeology, said Romanists across the country and farther afield were already discussing the significance of this first Exeter fort when the second came to light.
Suspicions that there was a fort on the site arose three years ago during trial trenching.
Excavations before the site was developed for a retirement village unearthed V-shaped protective ditches, which in places were more than two metres deep.
Now ditches on a different alignment to those of the first fort have been found, and Mr Gent said: “The new V-shaped ditch cuts through trenches that were dug to hold timbers for the first fort’s barrack blocks – these are long fairly narrow linear trenches. This shows that the army used the site again at a later date.
“It looks as if we now have three military establishments in Exeter – the known fortress in town, our new fort at St Loye’s, and now this.”