As World War II approached its end during 1945, the Soviet Army began its advance on Berlin. The Nazis, with their depleted ranks, dwindling supplies, and shattered morale following the suicide of their leader, were not in great shape. Still, they had a few things in their favor, one of which was the Flakturm Tiergarten, known in English as the Zoo Flak Tower.

After the British Air Force (RAF) had gone on its first bombing run against Berlin in 1940, dropping nearly 200 bombs in the process, Hitler began to reconsider how the Nazis would defend themselves against a competent air force. They tackled the question in two ways at once. First, they started targeting British cities in hopes of stretching RAF forces thin, and at the same time, they bolstered the capital city's internal defenses.

What emerged was what civic planners today would refer to as a "mixed use" building. Towers were built with both offensive and defensive capabilities. The Zoo Tower, built in 1941 and named because of its location close to the city's zoo, was among the most prominent of these as explained in this video about the towers from YouTube channel Real Engineering.

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The equivalent of a 13-story building made out of reinforced concrete, the Zoo Tower had anti-aircraft guns on its rooftops and a 85-bed hospital on the third floor. At the top, it featured four 12.8-cm twin FlaK 40 mounts, supported by 20 mm and 37 mm guns on lower platforms. A whole 26 feet (8 meters) of concrete protected the tower's sides and 16 feet (5 meters) protected the top. At full capacity, the Tower could hold up to 350 soldiers and 15,000 people.

Americans examining the anti-aircraft guns atop a German flak tower after the war. Life Collection Getty Images

As the Soviets rolled into town, the Zoo Tower's anti-aircraft guns were pointed towards the ground and the Red Army's tanks. While conditions in the Tower were hardly comfortable, with little room to breathe or water to drink, its defenses held. The guns kept the Soviets at bay, leaving them unable to advance on the Reichstag in open daylight.

Of course, a stationary tower with dwindling supplies and a limited reach can only do so much. Even as the Zoo Tower remained impregnable, the Soviets were able to fly their flag over the Reichstag. The Soviets allowed the troops inside to surrender, though Nazi command was able to delay the surrender until additional soldiers were able to escape from Berlin.

A few years later, in Allied-controlled Berlin, the Zoo Tower fell under British control. Remembering how the Tower had been built to frustrate their attacks, the British blew it up. This took three attempts with increasing amounts of dynamite. Eventually, 35 tons of dynamite were able to fell the Zoo Tower.

The site was eventually retaken by the Berlin Zoo and is the current home to the hippo exhibit.

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