Thor, with no disrespect but you are wrong. Security in depth does not
work and I am not planning to support my argument in any way. This is
just my personal humble opinion. I've seen only failure of the
principles you mentioned. Security in depth works only in a perfect
world. The truth is that you cannot implement true security mainly
because you will hit on the accessibility side. It is all about
achieving the balance between security and accessibility. Moreover,
you cannot implement security in depth mainly because you cannot
predict the future. Therefore, you don't know what kinds of attack
will surface next.
Security is not a destination, it is a process. Security in depth
sounds like a destination to me.