The coronation of Birger Jarlsson Valdemar took place in the cathedral in 1251; and in the reign of Gustavus Vasa several important diets were held in the town.
Already during his brother's lifetime, as duke of Schleswig, Valdemar had successfully defended Denmark against German aggression.
Immediately after his coronation, he hastened to his newly won territories, accompanied by the principal civil and ecclesiastical dignitaries of Denmark, and was solemnly acknowledged lord of Northalbingia (the district lying between the Eider and the Elbe) at Lubeck, Otto IV., then in difficulties, voluntarily relinquishing all German territory north of the Elbe to Valdemar, who in return recognized Otto as German emperor.
By closing Lubeck Valdemar had German trade and the German over-seas settlements entirely at his mercy.
Valdemar's position was still further strengthened when Frederick II., the successful rival of Otto IV., was, in 1215, crowned at Aix-la-Chapelle.