The remains of a Roman fort have been discovered below the Exeter Bus Station site.
Archaeologists have described the findings as "very important and unexpected".
The Roman ditch was first uncovered, then a further discovery was made of a previously unknown military site.
Coins and pottery made in the area for Roman soldiers were also discovered, along with fine tableware imported from France.
The find was made by the Exeter office of Cotswold Archaeology, who are working with Kier Construction ahead of the construction of a new bus station and leisure complex.
The council said archaeology work at the site, would not affect the construction timetable.
The present bus station was constructed in the early 1960s. At that time, there was no planning requirement for developers to record remains.