Hammershus is Northern Europe's largest medieval fortification, situated 74 metres (243 ft) above sea level on Hammeren, the northern tip of the Danish island of Bornholm in the Baltic Sea. Erected in the 13th century, it was long believed that the castle was built as a private residence for the archbishop of Lund. However, new evidence found at the ruins of the castle suggests it was constructed in the beginning of the century as a royal residence for Valdemar II of Denmark and a base for the Danish crusades, according to Kjeld Borch Westh, superintendent of the National Museum of Denmark.
The fortification consists of the base castle residence and accompanying Mantel Tower, and includes a great stonewall stretching 750 metres (2,460 ft) around the castle grounds. Bricks found at the tower during the renovation led Westh and other experts to revise their theory on when the structure was originally constructed, moving back the date from around 1255 to the beginning of the century, when Valdemar became king.