Rosenborg Castle in Copenhagen, Denmark, was originally built as a summer residence in 1606 during the reign of King Christian IV. It was only used as a royal residence for around 100 years, although it was briefly used as emergency housing in 1801.
King Christian IV loved his castle so much that he was brought there as he was dying, and he spent his final days there. Nowadays, the castle is best known for housing the coronation chair and the crown jewels.
It is a wonderful place to visit, and the gardens are quite enjoyable if the weather is nice out. Plan your visit here.