Flakturm IV St Pauli & Reeperbahn

Rising up from the northern reaches of St Pauli, this brooding concrete structure was built during WWII as an anti-aircraft defence position but doubled as a bunker or shelter for those fleeing bombardment – a rather strange combination when you think about it. Around 18,000 people could shelter here behind the 3.5m-thick walls. Now it's a nightclub, Uebel und Gefährlich.