14. Flak Towers
The Flak Towers were built by the Nazis during World War II as a method to defend Germany against air raids. According to historical records, the walls of the towers were over eleven feet thick and made of concrete, while many of the towers were built 10-20 stories tall. Many of the towers were built along the borders of Germany, usually outside of the major cities and had multi-level guns that could fire over 8,000 rounds per minute. The towers were also built to house up to 10,000 civilians and were complete with an underground hospital for tending to the wounded. Over the years, the towers have been re-purposed with two being completely demolished after the war; however, one tower stands abandoned to this day.