An enigmatic stone and turf structure on Bodmin Moor that was previously thought to be a medieval animal pen has been found to be 4,000 years older – and unique in Europe.

The rectangular monument was built not in the early medieval period to corral livestock, as recorded by Historic England, but rather in the middle Neolithic, between 5,000 and 5,500 years ago, archaeologists have discovered.

Nothing like it is known in Britain or farther afield, according to experts, meaning that the original purpose of the monument known as King Arthur’s Hall is a mystery.

“There isn’t another one of these anywhere,” said the lead archaeologist, James Gossip. “There is nothing built at that time or subsequently in prehistory that is a rectangular earth and stone bank with a setting of stone orthostats around the interior. There is no other parallel.”

The so-called “hall”, which sits on the western side of Bodmin Moor near Helstone in Cornwall, consists of a banked enclosure measuring 49 metres by 21 metres, lined on the inside with 56 standing stones up to 1.8 metres tall.

Cornwall National Landscape, which looks after the county’s protected land, commissioned the excavation after initial investigations by a group of local amateurs raised questions about its medieval attribution, Gossip said.

Through careful excavation and soil dating, using optically stimulated luminescence (OSL), members of the Cornwall Archaeological Unit and experts from the universities of Reading, St Andrews and Newcastle established that the interior of the monument had been dug away about 3000BC.

“They’ve dug down through the earth of Bodmin Moor to the loose granite on the surface, and they piled it up to make these ramparts. And what they did in our favour was they buried these very ancient soils below them which we could target for OSL,” Gossip said.

As for its intriguing name, which dates to at least 1583, the monument certainly wasn’t built by or for King Arthur, who – if he existed at all – is associated with the early Anglo-Saxon period in the fifth and sixth centuries AD.

“The middle ages was a period when the Arthur name starts being attributed to all sorts of unusual sites that the local population at the time probably didn’t understand,” Gossip said. “That suggests its original function had been lost by that point, but people attributed it to King Arthur because he had this association with something mythical and powerful.”

The middle Neolithic, which predates the stone circles of the bronze age, was a time when people were starting to settle in the same place for the first time and building enclosures, often on the high tors, he said. “The thinking is that these are meeting points for communities, perhaps to mark special occasions or to carry out ceremonies.”

His own favoured theory is that King Arthur’s Hall functioned in the same way, as a place for the community to gather. “It remains as an enigma, but we now know a little bit more about it, and we can firmly place it in the prehistoric landscape context of Cornwall.”