Aggersborg Aggersborg is the largest of Denmark ' s former Viking ring castles , and one of the largest archeological sites in Denmark . It is located near Aggersund on the north side of the Limfjord . It consisted of a circular rampart surrounded by a ditch . Four main roads arranged in a cross connected the castle centre with the outer ring . The roads were tunnelled under the outer rampart , leaving the circular structure intact . The ring castle had an inner diameter of 240 metres . The ditch was located eight metres outside of the rampart , and was approximately 1 . 3 metres deep . The wall is believed to have been four metres tall . The rampart was constructed of soil and turf , reinforced and clad with oak wood . The rampart formed the basis for a wooden parapet . Smaller streets were located within the four main sections of the fortress . The modern Aggersborg is a reconstruction created in the 1990s . It is lower than the original fortress . Dating of the site . Dating the structure has proven difficult , since the archaeological site has also been the site of an Iron Age village . The ring castle is believed to have been constructed around 980 during the reign of king Harold Bluetooth and / or Sweyn Forkbeard . Five of the six ring castles in historical Denmark have been dated to this era . The structure was completed within one or two years , and only used for a short period of time ; between five and twenty years . Archaeological remains . Archaeologists have estimated that the ring castle could hold a 5 , 000 - man garrison , located in 48 longhouse s . Twelve longhouses were located in each quadrant , all located on a north - south or west - east axis . No remains of the actual houses exist , but proof of the location of the walls has been found . The individual houses are believed to have been similar to the form seen on the Camnin chest , a house - shaped reliquary , as well as on house - shaped tombstone s in England . The houses had curved roofs and curved sides , similar to the form of a ship ; 32 . 5 metres long and 8 . 5 metres across . They were divided in a long inner hall , around 19 metres long , with smaller rooms at the end . It is estimated that construction of a single Aggersborg house required 66 large oak trees . The entire structure , housing included , is estimated to have used 5 , 000 large oaks . A large number of archaeological finds have been discovered on the site , including many imported luxury items . Examples include beads of mountain crystal and pieces of glass jars . A damaged golden ring has been discovered on the site as well ; a replica is displayed in the Aggersborg museum . Aggersborg was located both protected but also easily accessible by ship , since both ends of the Limfjord were open waterways when the fortress was constructed . It is speculated that ships had to be carried on land past Løgstørgrunde . Aggersborg was also strategically located at one of the three ancient crossings of the " Hærvejen " ( " army road " ) across the Limfjord . The two other crossings existed to the north of Farstrup , and near Lindholm Høje in Ålborg , respectively . No conclusive data yet exists whether Aggersborg was a stronghold controlling trade routes or whether its primary function was as a barracks / training grounds in connection with Sweyn Forkbeard ' s Viking raids on England . < br > Panorama of the Aggersborg site < br >