federal reserve bank ... the entire commercial "vault" was actually a steel cage.
I've spent some time inside federal grand jury evidence storage rooms and assisted in building one. (My assistance consisted mainly of vacating the space, but I digress...)
The slang term in our bureau was "slab to slab." The room had to have a steel-reinforced, poured concrete slab for both the ceiling and floor. The walls could be concrete or a steel cage such as you describe. Obviously, there were no windows and only one door.
What struck me as funny was that the HVAC routing was complex. It could be done by competent engineers, I suppose, but the one project I worked was fubar from the start. After the room was complete, the condensation inside was like a tropical rain forest. It took years and tons of mildewed documents before enough steps were taken to fix it.
Thanks for sparking that memory back to life in my brain. I guess I haven't yet lost all my facilities.