 Nonnebakken , a hill in the Odense area , is the site of one of Denmark 's six former Viking ring castles , built during the reign of Sweyn Forkbeard , who had forced his father Harold Bluetooth to leave the country and seek refuge by the Jomsvikings in Wollin ( modern Poland ) in around 975 . The fort provided its occupier the command of the Odense River passing next to the hill . The archaeological remains of the fort were heavily damaged when a building for the Odd Fellow lodge was constructed on the site during the late nineteenth century . 
