Not the way I'd do it. Defence in depth means securing everything. Starting with the perimeter, working inwards to individual apps - on both clients and servers. Every resource needs to be secured. That means spending cash, and the amount of cash that should be spent should be proportionate to the value of the asset being protected. If you have a server application or service, put an application firewall in front of it, so that both internal and external access goes through it. Don't just write a threat model, document the threat tree. Don't trust your employees, your software, hardware or building security. And don't trust the bosses either.

It's analogous to having a bodyguard. If you're in the bedroom and leave your bodyguard in the kitchen for a private conversation, the bodyguard and his big six gun are going to be of absolutely zero use when ninjas come crashing through the bedroom window.