Cologne, like many other former Prussian cities, is still surrounded by a ring of fortresses. 11 massive red-brick forts, built between 1873 and 1881, encircle the old city to this day. However, the Prussian fortification ring was never tested in battle. Instead, with the end of World War I, the city was faced with a legacy of disarmed fortresses which were to be demolished according to the Versailles Treaty. But thankfully then-mayor (and post-World War 2 chancellor of Germany) Konrad Adenauer struck a deal with the British occupation authorities, convincing them that he would remodel the existing military infrastructure in a way that would benefit both city and citizens. And his legacy to this day is the outer green belt, the Äußerer Grüngürtel, a ring of parks that surrounds Cologne in the west. Pleasant parts here include the Lindenthal canals, for example. 

One of my favourite spots in the green belt is the partially demolished former Fort VI, which today houses a sports club and band rehearsal spaces together with a nice stone garden open to the public, a pleasantly dark folly with the concrete and brick blocks of the fort overgrown by trees and weeds, turning the park into something that on a dark autumn day bears resemblances a much wilder place. Nearby Decksteiner Weiher west of the fort is an artificial lake created in the former field of fire, lined with trees on its banks and dotted with pleasure boats in summer. A fine peaceful place, built on the ruins of militarisation.