Copenhagen's Kastellet, or Citadel, was built by Frederik III to prevent invasion by the Swedes. Surrounded by grass-covered ramparts and a series of moats, it's one of the best examples of star-shaped fortresses in Europe, and was able to defend the city on all sides. The building itself is not open to the public, but the grounds are very popular with locals, and have wonderful views of the city and coastline. Admission is free.