Rolling Norfolk fields, where faint marks can be seen tracing the streets and houses of a buried Roman town, have been bought with English Heritage, National Heritage Memorial Fund and local authority money in an unusual move to preserve an archaeology site for ever in public ownership.

The name of Venta Icenorum, on the river Tas on the outskirts of the modern village of Caistor St Edmund, preserves the memory of one of the few local tribes the Romans had good reason to fear: the Iceni who, led in rebellion by their famous queen, Boudicca, torched the invaders' towns at Colchester and London in AD61.

Archaeologists believe the remains of the town are in serious danger from unauthorised metal detecting and intensive agriculture.

Only a few banks and fragments of stone walls remain above ground, but beneath the earth there are extensive remains of the Roman town where the mutinous Iceni eventually settled down to live in regularly planned houses and streets.

The crop marks also reveal the end of the straight Roman road from Colchester – so they could march straight up to crush any further stirrings of insurrection.

The site is particularly precious to archaeologists because most Roman settlements developed into modern towns and cities, so the remains have been destroyed by later foundations. Greenfield sites, such as Wroxeter in Shropshire and Silchester in Hampshire, are rare, and Venta Icenorum is particularly interesting because there is evidence that it evolved into a Saxon market town, before being abandoned to sheep.

A large part of the site, 22 hectares (55 acres), which until earlier this year didn't even have the protection of scheduled ancient monument listing, have been ploughed regularly as arable fields – and every time the land was ploughed, the footprints of unauthorised metal detectors were seen in the fields.

The land has now been bought with grants of £374,000 from the National Heritage Memorial Fund – a fund of last resort, which is administered by the Heritage Lottery Fund, but can move faster when a case is seen as urgent – along with £40,000 from English Heritage and £20,000 from South Norfolk council, and money raised by the Norfolk Archaeological Trust, a rare move to bring an archaeology site into public ownership, and the first time the NHMF has bought a site purely for its archaeological value.

The land will be added to the 49 hectares (120 acres) of the site acquired by the trust in the 1990s that is let for sheep grazing and interpreted by signs explaining the buried town to walkers.

"We believed the danger to the buried archaeology from ploughing and metal detecting was very real," Peter Wade-Martins, director of the trust, said. "Our priority will be to return the whole site to grass and gentle countryside enjoyment for the public."

The site will now be open to the public on both banks of the river – presenting more problems to visitors than the Iceni had, as the Roman bridge that connected the two parts of the town collapsed 1,500 years ago.

Will Fletcher, English Heritage inspector of ancient monuments for the area, said the chance to unite and conserve the site was "a once in a lifetime chance" to safeguard an important part of the nation's heritage.

Part of the site was excavated in the early 1930s when the first aerial photographs showed the buried structures, but most of it remains unexplored. A long-running research excavation project led by Will Bowden of Nottingham University, will resume at the site next month.