A Viking fortress newly discovered on Zealand, Denmark's largest island, might have served as a launching-off point for Viking attacks on England, the Telegraph reports. The fortress is shaped like a ring and is the fifth of its kind to be discovered. All five are in Denmark.
This, however, is also the first fortress of its kind that archeologists have found in more than 60 years, and, at 475 feet in diameter, it's the third largest ever discovered.
The team that discovered the fortress used a technique called gradiometry, which measured the planet's magnetic field to detect variations in soil patterns that might indicate past human tampering, Gizmodo reports. Researchers think that the fortress dates back to the tenth century, possibly during the reign of Harald Bluetooth, the Telegraph continues. Alternatively, it could be that it was actually Harald's son, Sweyn Forkbeard, who commissioned the fortress. Pending lab tests could narrow down the specific construction date and shed light on those questions.
Sweyn was the first Danish King of England, and the fortress could have acted as a training or holding facility for launching the attacks that won him that title, the Telegraph writes. The entire fortress has not been excavated yet, but experts suspect that Viking long houses or other structures once occupied the circular patch of ground behind the fort's protective walls. Remnants of the four massive wooden doors that once served as the only entry or exit, Gizmodo adds, are charred—likely they were burnt down during an attack.
"The Vikings have a reputation as a berserker and pirates," said Søren Sindbæk, a professor of medieval archeology at Aarhus University and one of the researchers behind the discovery, in a release. "It comes as a surprise to many that they were also capable of building magnificent fortresses."