The medieval tower of Kärnan standing in Helsingborg

Kärnan, meaning 'the core', is a medieval tower located in the city of Helsingborg, Scania, southern Sweden. It is the only part that remains of a larger Danish fortress which, together with Kronborg on the opposite bank of the Öresund, controlled the entranceway between the Kattegat and the Öresund, as well as the entrance to the Baltic Sea. Danish legend places the origin of the fortress in the reign of the legendary King Fróði, though this legend has not been supported by archaeological proof.

However, dendrochronological dating has confirmed that the core of the fortress was constructed in the 1310s, during the reign of Erik VI of Denmark. It was critical in securing control over the strait between Scania and Zealand. Despite this, the fortress was handed over to Sweden as part of the Treaty of Roskilde in 1658. It was retaken by Danish forces in 1676 during the Scanian War, with its capture celebrated by flying a giant Flag of Denmark above it.

The flag was later recaptured by the Swedish army and is preserved in the Army Museum in Stockholm. The fortress returned to Swedish control by the Treaty of Lund in 1679. Charles XI of Sweden ordered the demolition of most of the fortress, fearing a sneak attack from Denmark, but the medieval tower core was spared. The tower continued to serve as a navigational aid for ships passing through Øresund. In 1741, the tower was donated by the government to the city of Helsingborg.

The refurbishment of Kärnan was initiated in 1893–94 under the instruction of Oscar Ferdinand Trapp, a Swedish businessman and engineer, and under the guidance of Josef Alfred Hellerström, Helsingborg's city architect from 1903 to 1928. The objective of the refurbishment was to restore, to the extent possible, the appearance of the structure as depicted in the oldest known medieval illustrations. Kärnan has a roller coaster based on it, named The Oath of Kärnan, at Hansa-Park in Sierksdorf, Schleswig-Holstein, Germany.

Sources: Wikipedia, OpenStreetMap, Wikidata, Wikimedia Commons