A short entry in the Anglo-Saxon Chronicle describes how in 789AD Viking ships landed on Britain's shores and immediately killed a local official who welcomed them.

This was just the beginning of what would become a long history of vicious raids and fierce battles between the Norsemen and the English.

Now, archaeologists have uncovered a ring-shaped fortress that historians believe was used to train Vikings ahead of a large invasion of England in 1013.

The newly discovered fortress is large at 475ft (145 metres) from side to side and is the same shape as a typical Trelleborg plan, with four 'gates' like the one pictured above

Located in Vallø on the Danish island of Zealand - around 30 miles (48km) south of Copenhagen - the fortress is only the fifth of its kind to be discovered – and the first in 60 years.

‘The Vikings have a reputation as a berserker and pirates. It comes as a surprise to many that they were also capable of building magnificent fortresses,’ said Søren Sindbæk, professor of medieval archaeology at Aarhus University.

‘The discovery of the new Viking fortress is a unique opportunity to gain new knowledge about Viking war and conflicts, and we get a new chance to examine the Vikings' most famous monuments.’

Nanna Holm, curator of The Danish Castle Centre, said that precise new laser measurements of the landscape led her to the hidden circular fortress, which is large at 475ft (145 metres) from side to side.

Located in Vallø on the Danish island of Zealand (both marked) - around 30 miles (48km) south of Copenhagen - the fortress is only the fifth of its kind to be discovered – and the first in 60 years

Archaeologists (pictured) have uncovered a ring-shaped fortress that historians believe was used to train Vikings ahead of a large invasion of England in 1013

‘We recognise the “Trelleborg” fortresses by the precise circular shape of the ramparts and by the four massive gates that are directed at the four corners of the compass,’ Dr Holm explained.

WHAT IS A TRELLEBORG? Trelleborgs are ring-shaped Viking fortresses. They were all built in a circular shape with a rampart and four gateways. A total of five are known to exist - located in modern Denmark and the south of Sweden. The majority of trelleborgs have been dated to the reign of the Harold Bluetooth of Denmark who died in 986AD. The newly-discovered Vallø fortress is thought to date back to the late tenth century, and could have been built at the request of the king. But some experts think it was made by his son,Sweyn Forkbeard,who was the first Danish king of England. They say it could have been used as a military training camp or barracks from which to launch his invasions of England. King Forkbeard took control of London in 1013 and was soon afterwards declared to be King of England. Advertisement

‘Our investigations show that the new fortress was perfectly circular and had sturdy timber along the front; we have so far examined two gates, and they agree exactly with the “Trelleborg” plan. It is a marvellous find.’

Some historians think that the geometric design of the Trellenborgs - which are named after the location of the first one discovered - was inspired by old Roman army camps.

Dr Sindbæk said that the find was the result of considerable detective work.

‘We suspected that one fortress was 'missing' in the island Zealand.

'The location at Vallø was quite the right setting in the landscape: in a place where the old main roads met and reached out to Køge river valley, in the Viking Age was a navigable fjord and one of Zealand's best natural harbours. From there we worked our way forward step by step.’

Dr Holm said that the fortress was an active military camp and probably the scene of considerable fighting.

‘We can see that the gates were burned-down. In the north gate we found massive, charred oak posts.’

A short entry in the Anglo-Saxon Chronicle describes how in 789AD Viking ships landed on Britain's shore and immediately killed a local official who welcomed them –and this was just the beginning with more vicious raids and fierce battles to come, with London taken by Vikings (actors pictured) in 1013AD

Dr Sindbæk said: 'The location at Vallø (pictured) was quite the right setting in the landscape [for the fortress]: in a place where the old main roads met and reached out to Køge river valley, in the Viking Age was a navigable fjord and one of Zealand's best natural harbors. From there we worked our way forward step by step.’

HOW DID THE EXPERTS FIND THE FORTRESS? Helen Goodchild, an expert in archaeological geophysics from the University of York, measured small variations in the Earth’s magnetism to find features without damaging the site. Dr Sindbæk explained: ‘In this way we achieved an amazingly detailed “ghost image” of the fortress in a few days. Then we knew exactly where we had to put in excavation trenches to get as much information as possible about the mysterious fortress.’ Advertisement

She is sure the site dates from the Viking Age because of its distinctive design. Samples of the burned wood are being carbon dated to provide more accurate dating.

‘If we can establish exactly when the fortress was built, we may be able to understand the historic events, which the fortress was part of.’

Dr Holm is eager to find out whether the castle will turn out to be from the time of King Harald Bluetooth, like the previously known fortresses, or perhaps a former king's work.

‘As a military fortification from the Viking Age, the monument may help to unravel the position of Zealand in relation to the oldest Danish kingdom,’ she said.

So far, only small parts of the fortress have been excavated. Dr Sindbæk said: ‘This is really exciting. A find like this does not happen many times in his life-time.

‘The excavation has confirmed far more than we dared hope, but there is much more to learn.

‘The next big question is whether there were large buildings inside the castle, as in the known Trelleborg fortresses.