The huge pentagonal bastion-and-moat Kastellet was built to guard the entrance to Copenhagen’s inner harbor starting in 1626 under Christian IV, and expanded a few decades later by Frederik III. Even today, it’s still administered by the Ministry of Defence, but mostly used as offices. The rest of the citadel site, including the 1703 church, is open for visitors, and the tree-lined bastions and ramparts today provide some of Copenhagen’s most unique recreational space, full of joggers and picnickers taking in the harbor views. The Dutch-style 1847 windmill is particularly photogenic and even still works – some hobbyists put the sails on and grind flour from time to time.
Just beyond the fortress' southeastern edge is Anders Bundgaard's monumental Gefion Fountain, depicting the Norse goddess Gefion steering some rather stoic oxen.
