Which sequence correctly lists the three lines of defense from outermost to innermost?

• A. Perimeter barrier; interior controls; building exterior
• B. Building exterior; interior controls; perimeter barrier
• C. Interior controls; building exterior; perimeter barrier
• D. Perimeter barrier; building exterior; interior controls

Answer: (D)

Security is built in layers from outside to inside. The outermost line is the perimeter barrier, like fences and gates, which define the property and slow or stop intruders before they reach anything else. The next layer is the building exterior, which adds further defense with secure doors, reinforced walls, and exterior lighting to control entry and visibility. The innermost line consists of interior controls, such as restricted access areas, badge systems, alarms, and procedures that manage access once someone is inside. Therefore, the order from outermost to innermost is perimeter barrier, building exterior, interior controls. Moving interior-focused protections ahead of the outer boundary would undermine the purpose of deterring and delaying entry in the first place.