Kastellet is the last remaining part of a fort, which was built in 1662 and 1663 as a protection against Swedish soldiers. In 1725 its purpose was changed, and it had to serve as a prison. The most famous prisoner was Johann Friedrich Struensee, who lived in the 18th century. He was the personal physician of the Danish king, and the secret lover of the queen.
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Today the Kastellet is used by the Danish army for military purposes, and can be found in the district of Fredriksstaden (Churchill Parken).