Archaeologists in Denmark have found a ring-shaped Viking fortress on the island of Zealand, around 50km south of Copenhagen. It is the fifth circular fortress to be unearthed, and the first in more than 60 years.

Lasse Sonne, a Viking historian at the University of Copenhagen, said: "Although there were Vikings in other countries, these circular fortresses are unique to Denmark. Many have given up hope that there were many of them left."

Like previously discovered ring fortresses, the Vallo Borgring is thought to date back to the late 10th century and the reign of Harald Bluetooth, the King who christianised Denmark and Norway.

Some historians contend the fortresses were constructed by his son Sweyn Forkbeard, the first Danish King of England, as a military training camp or barracks from which to launch his invasions of England.

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Forkbeard seized London in 1013 and was declared King of England.

The newly discovered fortress has a diameter of 145m and consists of a 10m-wide circular rampart surrounded by a palisade of wooden spikes.

It appears to match the design of Denmark's other ring fortresses, sticking to a strict geometric pattern.

he other three are located in Aggersborg and Fyrkat in northern Jutland, and Nonnebakken near Odense.

The fortresses have four gates in different compass directions, and an interior courtyard symmetrically divided into four quarters.

It is thought that Viking "longhouses" would have been constructed within the fortress. Historians believe the geometric design may have been inspired by old Roman army camps found by Vikings during their raids on England.