During World War II, the Nazis populated German cities with these flak towers.

These complexes were often built with forced labor, and their purpose was to provide an impregnable base for anti-aircraft guns. Additionally, citizens could shelter within them during air raids. The concrete walls were up to 3.8m (12.5') thick.

Some of these flak towers were destroyed after the war by the Allies. But this 50-meter-tall tower in Hamburg, below, was too large to practically destroy; the amount of explosives required would've damaged the adjoining neighborhoods.

In the 1990s, local authorities transformed it into the Media Bunker, a music venue and cultural hub, hosting everything from music concerts to art exhibitions. Today there's a nightclub on the top floor, and now the building has received a green facelift.

It's now called the St. Pauli Bunker, or the Green Bunker:

"The visual highlight of this pioneering landscape architecture project is the spectacular public rooftop garden, which is likely unique to all of Germany. It has a fantastic panoramic view over Hamburg, is within eyeshot of the Elbphilharmonie, and has a planted "mountain path" that winds upwards around the outside of the bunker."

"For the first time, the bunker will also receive a memorial for the victims of the Nazi regime and the Second World War. Furthermore, rooms for local culture, exhibition areas, and a hall for sports and cultural events will be created."

This video gives you a look inside, and atop, the impressively massive structure. (It's narrated in German, but English subtitles are available.)

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