In 1203 it was conquered by Waldemar II., king of Denmark, but in 1227 it reverted to Albert, a son of its former duke.
Hadersleben is first mentioned in 1228, and received municipal rights from Duke Waldemar II.
About 1312 Frederick, who had become involved in a dispute with Waldemar, margrave of Brandenburg, over the possession of lower Lusatia, was taken prisoner.
In the 12th century the town was founded and a castle erected on Knudshoved (Canute's Head) by Knud, nephew of Waldemar the Great; and from the 13th to the 15th century Nyborg was one of the most important places in Denmark.
In the meanwhile, the conquest of Wisby by Waldemar IV.