The flak tower during world war 2 was used to defend Germany from allied air support. They were tough fortresses to disable, and could take multiple direct hits from bombers and still be fully functional.

This was an idea for my WW3 Base map, and I just took the schematic for it if you just wanted it and not the base.
The top has the flak guns, while the side holds the normal AA guns. There is an ammunition room, a Weapon room, 4 bunker, and a vehicle garage. The bunker lights can be turned off so that it is less likely to be detected at night.

This works best with a few of Flan's mod packs.
-A Random Assortment of Weapondry
-PaulaDeKoe's Pack
-WW2 gun packs: Germany Pack and Soviet Union Pack
-WW2 Pack
-Simple Parts Pack