[Nick Beale]

My initial understanding of the passage you quote is that these proposed facilities were NOT built as they would have used up a million cubic metres of concrete each.

If they didn't exist, that's why you (and I) haven't heard of them. I don't think the author is confusing anything with the facilities like Deelen. He says that the fortresses were to have permanent faciities for fighter control; tactical HQs; and search, homing and location devices.

There was a scheme underway in northern Italy for a small number of airfields with hardened facilities and very heavy Flak protection. There are some dcouments at Freiburg as I recall. The project never got very far but I think that the term "fortress" was used in this connection as well.

The philosophy? Well the Nazis had two main approaches in their repertoire: Plan A = slaughter defenceless people; Plan B = hide something under vast amounts of concrete. In this case they opted for Plan B and I guess they hoped to make their infrastructure bomb proof as with the U-Boat pens.