Common wisdom tells us that modern artillery and military aviation made use of stone fortifications obsolete. However, Flak Towers (Flaktürme) are a remnant of WWII that looks more like something from the time of the Crusades then age of the Blitzkrieg.
These monumental concrete fortresses were meant to serve as platforms for batteries of anti-aircraft guns. In order to boost the air defense of German cities, Hitler ordered the building of a series of immense towers throughout the country. Three of these towers were built in Berlin, additional two in Hamburg and, six more in Vienna.
The towers were not to be taken lightly, and were able to fire 8000 rounds per minute with a range of up to 14 km in a full 360-degree field of fire. Each tower complex consisted of two separate towers, one of these 8000 a minute G or gun towers and an L-tower (or command tower) which served as command center. In addition, the towers served as air raid shelters for up to 10, 000 people.
The tower walls were 3.5m of reinforced concrete, enough to survive an attack by conventional bombs carried by allied bombers of the age, but unlikely to survive the attack by so-called Grand Slam bombs especially designed for busting enemy bunkers.
Flakturm VII in Augarten, Vienna is a Generation 3 flak tower and was dangerous enough to be generally avoided by allied air-force during the war. Both G and L towers of Flakturm survived the war with little damage. After the war, because of their massively sturdy construction, destruction was deemed unfeasible. The towers remain disused to this day, and serve as a home to a colony of several thousands of pigeons.