The Rosenkrantz Tower was one of the most prominent buildings of Bergenhus Fortress. The tower derives its name from governor Erik Rosenkrantz, during who's reign (1559–1568) the tower received its present shape and structure. The oldest part of the building, however, is made up of a medieval tower, known as the "Keep by the Sea," built by king Magnus the Lawmender in the 1270s as part of the royal castle in Bergen.