Which statement best describes defense in depth in physical security?

• A. Layered controls across deterrence, detection, and response to reduce risk even when one control fails.
• B. A single, highly visible barrier deterring all threats.
• C. Relying solely on trained personnel to respond to incidents.
• D. Installing one high-tech system and assuming it covers all risks.

Answer: (A)

Defense in depth means using multiple layers of security controls that cover deterrence, detection, and response, so a failure in one layer doesn’t leave you exposed. In physical security, you combine visible deterrents like lighting and barriers, with detection such as alarms and cameras, and a prompt response from guards or incident procedures. Together, these layers create overlap and redundancy, reducing risk even if one control is bypassed.

That concept is captured by the statement describing layered controls across deterrence, detection, and response to reduce risk even when one control fails. A single, highly visible barrier isn’t enough because it relies on one layer and a breach could happen without other protections. Relying solely on trained personnel overlooks the need for ongoing deterrence and automated detection. Installing one high-tech system and assuming it covers all risks creates a single point of failure and ignores other threat vectors.