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In page Koldinghus:

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In 1252 Abel suddenly died during an expedition to Friesland leaving the kingdom without a leader. The natural order of things would be to elect Valdemar as king however he was in a prison cell in Cologne. This prompted the election of Abel's brother, Christoffer I, as the new king. Valdemar was released one year later when his family had finally succeeded to collect the funds for his ransom. Upon his return to Denmark Valdemar immediately challenged Christoffer for the throne but found little support. It was finally agreed that Valdemar would become Duke of Schleswig. Several wars between the king of Denmark and the Duke of Schleswig ensued until it was finally decided to build a fortress to defend the southern borders against its troublesome neighbour.