Brief synthesisThe ring fortresses of Aggersborg, Fyrkat, Nonnebakken, Trelleborg and Borgring, constructed between about 970 and 980 CE during the reign of King Harald ‘Bluetooth’ Gormsson, represent outstanding examples and technological mastery of military architecture. In most cases, they were equipped with a concentric ditch, axial streets encircled by a ring street, and rows of longhouses geometrically arranged in the four quadrants of the fortified ring. The ring fortresses testify to Harald’s state-building ambitions and can be seen as an outstanding testament of the process of state formation and an expression of a cultural shift in the geo-cultural context of Scandinavia and Northern Europe. The precise manner in which all five ring fortresses were built over a short period of time testifies to the existence of centralised power that was required to manage such a monumental infrastructure project involving resource-intensive engineering. AuthenticityThe original forms, designs, materials and substance of the ring fortresses have survived unaltered below ground at all five component parts, even in areas where archaeological excavations have taken place.