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Photo: derivative work: Mussklprozz (talk) Schloss_Gottorf_0728.jpg: Arne List, CC BY 3.0 via Wikimedia Commons
Houses two museums these days.
Gottorf Castle is a castle and estate in the city of Schleswig, Schleswig-Holstein, Germany. It is one of the most important secular buildings in Schleswig-Holstein, and has been rebuilt and expanded several times in its over eight hundred years of history, changing from a medieval castle to a Renaissance fortress to a Baroque palace.
© OpenStreetMap contributors
St. Peter
Haithabu
The remains of an ancient Viking settlement, which is now considered the most important archaeological site of Schleswig-Holstein. There is a museum and reconstructed houses where you can learn about the history of Hedeby and the everyday life of the Vikings.
Literally meaning the Earthworks of the Danes, the first of these walls were constructed already around 500 AD as a protective wall against the south. A few centuries later, it was expanded by the Vikings into a system of trenches and walls. Interestingly, the Danevirke was used in the war against Prussia in 1864 when the Danes lost the area.