Hierarchy classification:

1. top of the aat hierarchies
2. Objects Facet
3. Built Environment (hierarchy name)
4. Single Built Works (hierarchy name)
5. single built works (built environment)
6. [single built works by specific type]
7. [single built works by function]
8. fortifications (built environment)

Anotations

Scope note:

Large anti-aircraft gun blockhouses used by the Luftwaffe in World War II to protect against Allied overflights, serve as air-raid shelters, and to coordinate air defence. They had concrete walls up to 3.5 meters thick and were considered invulnerable.