definition
strict definition
air raid defense
A ground-based defensive position designed to engage attacking aircraft, including flak towers and gun emplacements.
An air raid defense position is built to engage attacking aircraft from the ground. Flak towers and gun emplacements belong here.
It shoots back, and that is what separates it from a shelter. A structure built to protect people during a raid is a different use, even where both stand on the same site.
Many of these outlast the war they were built for. The flak towers in Hamburg and Vienna still stand because their concrete is impractical to demolish, and several now hold other uses inside the original shell.
