Today’s lesson: German Air Defense Towers of WWII
During WWII, the Germans built a series of large, concrete anti-aircraft blockhouses in key cities in an effort to combat overflight of the cities by Allied aircraft. Because of their stout construction, many still exist today.
There were three generations of towers. They featured a gun tower (G-Tower) and a control tower (L-Tower). There were three generations of G-Towers.

And here are the survivors (on any Google Maps shots, click to go to Google Maps):
Berlin
Flakturm I (both towers destroyed at the end of WWII)

Flakturm I G-Tower

Flakturm I G-Tower
Flakturm II (L-Tower destroyed, G-Tower mostly destroyed)
Flakturm III (both towers partially destroyed)
Hamburg
Flakturm IV (G-Tower converted into a residential building, L-Tower destroyed)

Flakturm IV G-Tower
Flakturm V (L-Tower destroyed)

Flakturm V G-Tower

Flakturm V G-Tower
Vienna
Flakturm VI (G-Tower intact, L-Tower converted into an aquarium)

Flakturm VI G-Tower

Flakturm VI L-Tower
Flakturm VII (both towers intact)

Flakturm VII G-Tower

Flakturm VII L-Tower
Flakturm VIII (both towers intact)

Flakturm VIII G-Tower

Flakturm VIII L-Tower
Want to know more? Go here.