Archaeologists uncovered four ancient ring-shaped fortresses in Denmark in the 1930s and only recently discovered another:
Q: How did you discover the fortress?
A: It’s a bit of a detective story. I’ve been working with these ring fortresses for quite some time, and I came to the conclusion that their distribution didn’t make sense. There were gaps in the network of known fortresses where logically another fortress should have been. I went out looking for landscape features that matched those of the fortresses we knew already, namely accessibility to land and water routes. There were only a few locations in Denmark that really fit the pattern. The Danish state has made a high-resolution LIDAR image of the whole country, so we searched that and found this very, very big feature.
Q: How did it go unseen for so long?
A: The agricultural activity around it was extremely destructive. For hundreds of years throughout the Middle Ages, peasants ploughed and leveled the field. When we came, the fortress’s ramparts were less than half a meter above the average level of the field. You could walk the field, and I might have a hard time convincing you there was anything at all, but the LIDAR image was decisive.
Q: Why did Vikings build ring-shaped fortresses?
A: The ring is the perfect shape for a fortress. It’s the shape that encompasses the greatest area within the smallest circumference. But there’s no need to make it a perfect circle, and that’s what distinguishes the Viking Age ring fortresses in Denmark. Clearly the person who built these Viking ring fortresses—and we think that was King Harald “Bluetooth” Gormsson [who united Scandinavia, converted the Danes to Christianity and, more recently, lent his name to Bluetooth wireless technology], whose father was the first ruler of the Danish kingdom—wanted something more. All the fortresses share this strict geometry. Somebody with magnificent land-surveying skills was involved in this building work for no other reason than sheer prestige and to signal command and ability.
Q: Did the they invent the ring-shaped fortress?
A: No, they probably learned it from their own invasions in England. The people there built a network of fortifications about 100 years before our structures as a defense against the Vikings. It worked so well that the invaders could not get a foothold and had to turn back. It was a huge success for the Anglo-Saxon kings. So we believe that when ring fortresses then pop up in Denmark, it’s a copying of that strategy.
Q: Why are these structures important?
A: These ring fortresses have been the biggest mystery in Viking archaeology since the 1930s. People couldn’t believe the Vikings in their own country built these structures. They thought foreign armies must have built them. But as we found more of these, we found it was indeed a Danish king and his Viking warriors, and for that reason they have been part of the most fundamental reassessment of what the Vikings were all about. They were warriors, obviously, but they were warriors out of a very organized society.