The concept of using barriers arranged in concentric layers with the level of security growing progressively stronger as one approaches the center is referred to as:

Defensible space is a concept that focuses on the design and layout of communities to reduce crime through natural surveillance and territorial reinforcement. While it promotes safety, it does not specifically describe the layered barrier approach that "protection in depth" entails.

CPTED is a broader strategy that employs environmental design principles to reduce crime opportunities, emphasizing elements like natural surveillance and access control. However, it does not specifically address the concentric layering of security barriers that characterizes "protection in depth."

Zone protection refers to strategies that create different security levels across areas, but it lacks the specific focus on concentric layering of barriers and progressively stronger security measures that define "protection in depth." It is more about general area security rather than layered defenses.