If you look at a map of Copenhagen, you'll see what looks like a big star-shaped park above the city center. That's Kastellet, aka the Citadel. Built in the 17th century, it's shaped like a pentagon with cool bastions at each corner. Inside, you'll find a mix of military stuff, a church, windmill, and even a prison. It's a really cool mix of historic, interesting, and also a really cozy and beautiful park!