How the Roman fortress at Caerleon would have looked, according to archaeologists at Cardiff University. Video courtesy of 7Reasons 7reasons

Archaeologists have discovered only the second known port of Roman Britain, where soldiers would have arrived in numbers from the Mediterranean to aid in the fight against some of the most stubborn and hostile of all the tribes they had to face.

Over the last year, archaeologists have been digging near the Roman fortress of Caerleon, just north of Newport, south Wales, and have made some remarkable discoveries. On Tuesday, the site was declared the only known Roman British port outside London.

"It is extremely exciting," Peter Guest, leading the excavation team from Cardiff University, said. "What we have found exceeds all expectations. It now seems clear that we're looking at a new addition to our knowledge of Roman Britain."

Guest said the archaeologists had discovered far more than a quayside or harbour installation, adding: "It seems to be a deliberately founded and made port structure that goes with the legionary fortress in Caerleon."

The remains are incredibly well-preserved, partly because the land has been used for grazing for so long and has not been intensively ploughed.

Archaeologists digging on the banks of the River Usk have found the main quay wall as well as landing stages, wharves and dockside tracks.

The port would have been for the fortress, the farthest flung of all Roman outposts and the place where, some believe, Arthur later convened his Camelot court. The fortress was constructed in AD74-75 as the military headquarters of the second Augustan Legion, one of four legions that invaded Britain during the reign of the emperor Claudius.

Having a port on the River Usk would have made it far easier to supply the frontline than "traipse over mile after mile after mile of bumpy Roman road", Guest said.

Two thousand years ago, the locals in the area were the Silures, a tribe of ancient Britons who managed to keep the Romans at bay for a generation.

The Romans had a tricky time in south Wales, with the senator and historian Tacitus noting how fearsome, warlike and difficult to subdue the tribes there were. He described a struggle of nearly 30 years in which the locals skirmished and avoided full-on battle before they were finally pacified.

The port discovery tops a list of amazing finds made during the excavation of a suburb of large public buildings over the last year, with bath-houses, marketplaces and temples all having been unearthed.

The dig ends on 1 September, and the area is open to the public until then.

• This article was amended on 31 August 2011. The original referred to "Welsh tribes" existing at a time when there was no Wales. This has been corrected.