A massive tenth-century AD ring fortress was recently identified at Borgring, south of Copenhagen in Denmark. The combination of high-resolution LiDAR mapping, geophysical survey and targeted small-scale excavation has demonstrated that the site belongs to a rare class of monuments—the Trelleborg-type ring fortress. Borgring is the first such monument to be found in Denmark in over six decades, and provides an opportunity to investigate a type-site of Viking Age military organisation and conflict. The authors argue that Borgring complements a varied group of fortification structures in late Viking Age Denmark, part of a military network close to contemporaneous European ideas of military kingship and defence.