In a field southwest of Copenhagen, beneath a barely perceptible rise, lies what’s left of a fortress that may have been built by Harald “Bluetooth” Gormsson, the tenth-century Viking warrior who became the first king of Denmark. This is the first such fortress to be found in the country in 60 years. Archaeologists used remote-sensing surveys to identify the 475-foot-wide circular structure as well as buildings and pits in and around it. They also excavated two of the fort’s gates and found burned timbers in both, which could suggest that the fortress had been attacked. It is also possible that it was burned when it was no longer needed. There are four other known Viking fortresses in Denmark, all dating to around A.D. 981, during the reign of Bluetooth, and they all have the same layout. This one is providing a new opportunity to learn about the reign of the king who Christianized Denmark and implemented a national government. “It was an amazing time,” says Nanna Holm of the Danish Castle Center. “It is when we became who we are today.”