[lmm]

Every step is easy once you think about it; the hard part is spending any attention on it in the first place.

I would definitely say it's more effective to test your existing layers before adding more layers, and I think the "defence in depth" concept leads people astray there. Having multiple porous layers works on a battlefield where attacks are costly; it doesn't work on the internet where if one attack gets through an outer layer then all the other attacks can immediately get through the same way and start hitting the inner layer.