Any Level can be breached, the next layer below can't simply "fall over".
However, not all attacks flow nicely from the perimeter, so don't rely on the "layer above".
Perimeter: Guarding every access point to your network.
Network: Blocking all traffic that is not necessary on your network.
Host: Clearly identifying the purpose and operation of each server, securing each desired service and remove all others.
Application: Making consistent use of well-known corporate security sub-systems.
Data: The true asset to the organization, needs to be recoverable and protected as appropriate per device.
Physical: Physical attacks or theft can thwart all other defenses.
and a Policy to clearly define it!
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