Archaeologists from Aarhus University and the Danish Castle Centre have discovered the remains of a huge circular Viking fortress on the Vallø Estate, about 30 miles south of Copenhagen on the Danish island of Zealand. Only seven of these ringed fortresses have ever been discovered, all of them in Denmark or the southern tip of Sweden, and the last one was found 60 years ago. Its design is in keeping with the Trelleborg-type fortresses built by King Harald Bluetooth in around 980 A.D.
Much of this discovery was done in the research phase. Archaeologists suspected there was another fortress on Zealand. The Vallø site was a likely candidate because it was in a place where old Viking roads met close to the Køge river valley which in Viking times was a navigable fjord with one of the island’s best natural harbors. This made it an ideal setting for large military installation.
The team did a detailed laser survey of the site, measuring the minute features of the landscape. They found that a mound that was barely visible to the naked eye had a distinct circular outline. To investigate further before attempting excavation, they called in Helen Goodchild, an archaeological geophysics expert from the University of York, to do a magnetic survey of the site. Geomagnetic data derived from measuring variations in the magnetic field of the soil identified the archaeological features of a circular fortress.
The image from the geomagnetic survey revealed a massive structure 475 feet in diameter, which makes it the third largest of the Trelleborg-type fortresses after Aggersborg (787 feet in diameter), in Limfjorden, Denmark, and Borgeby (492 feet in diameter) near Lund in Scania, Sweden. The inner ramparts are 35 feet wide and circular, surrounded by a spiked palisade. Four gates are placed at the cardinal points of the compass. This is the same plan as the other Trelleborg-type fortresses.
Armed with this key information, archaeologists chose to dig in the areas most likely to produce information about the fortress as quickly as possible. The first trenches were dug at the site of two of the four gates. Both of them were burned down at some point. Large charred oak timbers were found at the north gate, a precious find both because the sturdy timber gate is another feature of the Trelleborg fortresses and because they’ll give us a precise date for the fortress.
Nanna Holm underlines that the fortress was a genuine military facility, and probably the scene of fighting as well. She’s in no doubt that it dates back to the Viking Age.
“Fortresses built like this one were only built in the Viking Age, and the burnt timber in the gates enables us to fix the date using radiocarbon dating and dendrochronology. We’ve sent off samples for analysis, and the result should be available in a few weeks’ time. The date will be vital. If we can establish exactly when the fortress was built, it will help us to understand the historical events with which it was connected.” […]
“We can’t wait to find out whether the fortress dates back to the time of Harald Bluetooth, or whether it was built by a previous king. A military fortification from the Viking Age may shed more light on the links between Zealand, ancient Denmark and the Jelling dynasty – as well as teaching us more about the period during which Denmark became Denmark,” says Holm.