The new fortress, called Borgring, was found principally using an aerial, laser-based surveillance method called LIDAR, which returns an extremely high-resolution 3D ground map. It’s located on the Danish island Zealand, south of Copenhagen. The stronghold is a perfect circle with an outer diameter of 144 meters, and has four main gates crisscrossed by wood-paved roads. The outer ramparts were built from earth and timber. Counting tree rings from its timber reveals that, like its cousins, it was built sometime in the 970s or 980s.