Aggersborg. Viking-Age Ring Fortresses tells the fascinating story of one of the five most important monuments of the Vikings in Denmark. The ring fortresses were built under King Harald Bluetooth in the late 900s. This book follows the archaeologists who uncovered the history of the largest of the fortresses – Aggersborg. In 1945 a researcher sat studying an archaeological survey at the National Museum in Copenhagen. The circular earthwork on the plan looked exactly like the strange Viking Age fortress that had been excavated at Trelleborg ten years earlier, but this stronghold was twice as large. Excavations showed that the earthwork on the plan was indeed a ring fortress like Trelleborg. When the 'new' ring fortress Aggersborg was discovered, one of the greatest mysteries in Danish history grew even bigger. What was the purpose and why were they built? Aggersborg is placed in North Jutland close to Norway, and this may be the key to size of this circular giant.