Archaeology students learning how to use mapping equipment have stumbled across the site of large Roman buildings on the banks of the river Usk in Wales, right by one of the best-known and most-studied Roman sites in Britain.

The structures have yet to be excavated, but one is enormous, possibly a granary or warehouse – or a palatial riverside villa.

The students located the previously unknown buildings as they were learning to use geophysical tools, which can reveal the outlines of buried structures, in fields by the Roman fortress at Caerleon – claimed by some romantics as King Arthur's Camelot. The area has been excavated and studied for two centuries.

The buildings lie outside the fortress walls, where archaeologists believed there was nothing except a few outbuildings and stores.

Cardiff University, whose students of the school of history, archaeology and religion made the discovery, has created a fly-through animation, which contrary to the old guidebooks and maps, now shows buildings stuffed in between the fortress and the river, including a huge rectangular complex surrounding a courtyard the size of a parade ground.

Dr Pete Guest, senior lecturer in Roman archaeology at the university, described the discovery as "completely new and totally unexpected".

"It is difficult to be certain about what we have found because nothing like this has been discovered in Roman Britain before. The buildings' ground plans suggest that they were of some importance. We think that they could have included markets, administrative buildings such as town halls, bath-houses, store buildings, or even possibly temples.

"The biggest is enormous and must be one of the largest buildings known from Roman Britain. We can only guess what it was for, but at the moment we're working on the idea that it had something to do with a harbour on the river, although it does look uncannily like a residential villa building – if that's the case it was built on a palatial scale."

Caerleon, Chester and York are the only three known permanent legionary forts, but the others are much harder to excavate because most of the remains are buried under the modern cities. In Caerleon, almost the entire site is still in open ground, though many of the 17th and 18th century buildings incorporated stones borrowed from the Romans.

More answers may emerge in the next weeks, as the students join staff and a team from University College London, in a six-week dig.

The dig, which will continue until mid-September, will be open to the public with daily tours, and displays of finds. The excavation will be updated regularly at the Council for British Archaeology's website.