Around 1150, a castle is built on the northern part of island Als as protection against the Wends. King III Sweyn name it Alsborg a name it keeps for the next 20 years. Around 1168, the problems with the Wends ravaging is back, this time on the southern part of island Als. That is why King Valdemar the Great buil a castle at the southern passage to the sound Als Sund. The new castle is named Søndre Borg and Alsborg is renamed Nørre Borg.
There were no cities. The castles were placed out of a strategic point of view; they had to be easy to defend. The cities Købingsmark, later renamed Nordborg and Sønderborg sprouted under the protection of the castles.