Traces of what used to be a Viking fortress had recently been discovered by archeologists in Denmark. Located about 30 miles southwest of Copenhagen, the circular fortress, called Vallø Castle Ring, is reminiscent of the ones built by King Harald Bluetooth around 980 AD, famously known as the Trelleborg fortresses.

"This is the first time for more than 60 years that a new Viking ringed fortress has been discovered in Denmark," said archeologist Nanna Holm, a curator at the Danish Castle Centre.

The site is located west of Køge, in the fields of Vallø Estate. As explained in the Danmarks Borgcenter website, Køge was a strategic location where main roads converged during the Viking age, and it also led to the Køge river. The river, at the time, was one of the area's natural harbors.

Like the Trelleborg fortresses, this one is also a perfect circle. The previous finds were equipped with massive gates which faced north, south, east, and west – the researchers already discovered two in their latest find.

According to Holm, the fortress, which measures 145 meters in diameter, was used as a military installation with earthen embankments, heavy oak posts, wooden gates, and massive timbers. The researchers also believe that it was used for fighting, as indicated by the charred remains.

"We can see that the gates were burned-down; in the north gate we found massive, charred oak posts," she said. Fortresses constructed in this manner are only known from the Viking Age. The burned wood in the gates will make it possible to determine the age by means of radiocarbon dating and dendrochronology."

The team initially suspected that a fortress was "missing" in Zealand and, after some careful deduction, they inferred that one was located in Vallø. With the expertise of Helen Goodchild (University of York) in archeological geophysics, they were able to survey the land using non-invasive techniques.

"From this survey, we knew exactly where we had to put in excavation trenches to get as much information as possible about the mysterious fortress," she said.

The discovery is an exciting find for Danish historians. They are eager to find out if it was built during the time of King Harald Bluetooth or of an earlier king, and it will give light to the role of Zealand to the medieval Danish kingdom.

"The Vikings have a reputation as berserkers and pirates," said Søren Sindbæk, a professor of medieval archeology at Aarhus University and Holm's colleague in the excavation. "It comes as a surprise to many that they were also capable of building magnificent fortresses."

He added, "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."