During the Kalmar Union, many castles and fortresses in Sweden were damaged as a result of the ongoing conflicts between Danes and Swedes. When Gustav Eriksson became Swedish king and the union was dissolved, he, together with the sons that followed him on the throne, invested huge sums of money in repairing these buildings. It was Gustav's son John III (r. 1568–1592) who ordered the reconstruction of Borgholm into a renaissance castle. During his reign, the Pahr brothers (four engineers and architects from Milan) led a significant rebuilding that took place from 1572. The castle acquired a Gothic character and became exemplary of the Italianate bastion style.