Although you might think that this is an old method and it doesn't apply to today's demands, the reality is that it still does, although you won't be using the same technologies that you used in the past. The whole idea behind the defense in depth approach is to ensure that you have multiple layers of protection, and that each layer will have its own set of security controls, which will end up delaying the attack, and that the sensors available in each layer will alert you to whether or not something is happening. In other words, breaking the attack kill chain before the mission is fully executed.
But to implement a defense in depth approach for today's needs, you need to abstract yourself from the physical layer, and think purely about layers of protection according to the entry point. Let's use the following diagram as an example of how defense in depth is implemented today:
The attacker has broad access to different resources. They can attack the infrastructure...