Cornelis Altenan was a Danish-German builder. He was employed by Christian III of Denmark

Christian III of Denmark

Christian III reigned as king of Denmark and Norway. He was the eldest son of King Frederick I and Anna of Brandenburg.-Childhood:...

 as a bricklayer and carpenter, and was responsible for building the King's fortress in Holstein

Holstein

Holstein is the region between the rivers Elbe and Eider. It is part of Schleswig-Holstein, the northernmost state of Germany....

.

He was also known as the king's architect and helped design Copenhagen Castle

Copenhagen Castle

Copenhagen Castle was a castle on Slotsholmen in Copenhagen, Denmark, built in the late 14th century at the site of the current Christiansborg Palace....

shortly before his death. He died though on 19 September 1558, when a Flensburg magistrate convicted him in a succession battle between his widow and his daughter Gertrud of first marriage.