Kärnan is a medieval tower, the only part remaining of a larger Danish fortress which controlled the entrance to the Baltic Sea.
The origins of the fortress is disputed but Danish legend places its origin to the reign of the legendary King Fróði. However, this legend has not been supported by archaeological proof. Dendrochronological dating has shown that the core was built in the 1310s, when Eric VI of Denmark was King of Denmark. It was considered the most important fortress in Denmark, and was integral in securing control over the strait between Scania and Zealand.
It was surrendered to Sweden along with the rest of Skåneland as part of the Treaty of Roskilde in 1658. The fortress was retaken by Danish forces in 1676 during the Scanian War, and its capture celebrated by flying a giant Flag of Denmark above it. This flag was later captured by the Swedish army and is preserved in the Army Museum (Armémuseum) in Stockholm. The fortress returned to Swedish control by the Treaty of Lund in 1679. Charles XI of Sweden ordered most of it demolished fearing that it was too exposed to a sneak attack from Denmark. The only thing that was saved for posterity was the old medieval tower core. The tower continued to serve as a landmark for shipping through Øresund.
The castle was restored starting during 1893-94, under instructions from Oscar Ferdinand Trapp, a Swedish businessman and engineer (1847–1916). Architect for the restoration was Josef Alfred Hellerström (1863–1931), Helsingborg city architect from 1903 to 1928. The objective of the restoration was to restore, to the extent possible, the appearance the structure had based upon the oldest known medieval illustration. The building's crenellation dates from these repairs.
References:Rosenborg Palace was built in the period 1606-34 as Christian IV’s summerhouse just outside the ramparts of Copenhagen. Christian IV was very fond of the palace and often stayed at the castle when he resided in Copenhagen, and it was here that he died in 1648. After his death, the palace passed to his son King Frederik III, who together with his queen, Sophie Amalie, carried out several types of modernisation.
The last king who used the place as a residence was Frederik IV, and around 1720, Rosenborg was abandoned in favor of Frederiksborg Palace.Through the 1700s, considerable art treasures were collected at Rosenborg Castle, among other things items from the estates of deceased royalty and from Christiansborg after the fire there in 1794.
Soon the idea of a museum arose, and that was realised in 1833, which is The Royal Danish Collection’s official year of establishment.