Kärnan is a medieval tower in Helsingborg, Skania, in southern Sweden. It is the only remaining part of a large Danish fortress that together with Kronborg Fortress opposite Oresund controlled the entrance between Kattegat and Oresund and further south of the Baltic Sea.
The origin of the Helsingborg fortress is disputed, but Danish legend says that its origins date back to the reign of the legendary King Frury. However, this legend has not been backed up by archaeological evidence. The dendrochronological dating shows that the core was built in the 1310s, when Erik VI was King of Denmark. It was considered the most important fortress in Denmark and was an integral part of the control of the strait between Scania and Zealand.
It was surrendered to Sweden along with the rest of Scaneland under the Treaty of Roskilde in 1658. The fortress was re-captured by Danish troops in 1676 during the Scandinavian War and its seizure was marked by the raising of a giant Danish flag over the fortress. This flag was later captured by the Swedish army and is kept in the Army Museum (Armémuseum) in Stockholm. The fortress returned to Swedish control under the Lund Treaty in 1679. Charles XI ordered the demolition of most of the fortress for fear that it was too vulnerable to attack by Denmark. The only thing that survived for posterity was the old medieval core of the tower. The tower continued to serve as a reference point for navigation through Oresund.
The reconstruction of the castle began in 1893-94 on the instructions of Swedish businessman and engineer Oskar Ferdinand Trapp (1847-1916). The architect was Josef Alfred Hellerström (1863-1931), city architect of Helsingborg from 1903 to 1928. The aim of the restoration was to restore the appearance of the building to the extent possible, based on the oldest known medieval illustration. The curving of the building dates back to these restorations.