Probably it's a ventilation shaft - because I would decipher the letters as "Bei Gasalarm schließen" means "To be closed at gas raid".

I work with real people (survivors...) who used to live here and in nearby associated institutions. These images certainly reflect at least the sense of isolation that I have heard ex patients talk of. The decay contrasted with the echoes of the opulent architecture captured in your pictures seems to mirror the contrast within society in that time of those within and those outside the perimeter.
Hauntingly evocative and for me providing a most graphic reminder of what those I support have lived through.
Awesome, in every sense....