Kastellet, Copenhagen
This is the 341-year-old Kastellet in Copenhagen, Denmark – one of the best preserved fortifications in Northern Europe. King Christian IV originally had the star-shaped island built to house a castle for him to hide in (should the need arise), but due to “economic constraints” the castle was never built. Today the island functions as a military area and has a museum, park, its own church and even a windmill… this is Denmark after all! There’s some more info at Wikipedia. Incidentally, this is also the site of the Little Mermaid, but it’s too small to see anything…