Knock John sea fort, one of several floating "Maunsell Forts," located in the Thames Estuary off the coast of England. Description from pinterest.com. I searched for this on bing.com/images
Nazi Flak tower. Used against allied aircraft in Europe. They had an air defense range of 14km and could put up 8,000 rounds in a minute!!!
Tiergarten Flak Tower (Flakturm Tiergarten) with twin 12,8 cm Zwillingsflak 44, Berlin, 1945. Read more: http://histomil.com/viewtopic.php?f=338&t=3918&start=4760#ixzz3Vh0uyaW3
Berlin WW2 German flak tower - there were three of these giants in Berlin,the Soviet tank guns could not dent them. They held thousands of Berliners and were surrendered at war's end.
Léopold Lambert re-envisions a decommissioned military space
On top of the Tiergarten flak tower.
Nefertiti in the Flak Tower | clivejames.com
10 Abandoned Gun Emplacements, Artillery Batteries and Flak Towers