The defense-in-depth principle is a strategy of applying multiple layers of security controls to protect an asset from threats. It is based on the assumption that no single security measure is sufficient to prevent all attacks, and that each layer adds more protection and reduces the risk of compromise. One example of applying the defense-in-depth principle is implementing alerts for unexpected asset malfunctions, which can indicate a potential security breach or incident. References: Cisco Cybersecurity Operations Fundamentals, Module 1: Security Concepts, Lesson 1.1: The CIA Triad and Security Concepts, Topic 1.1.4: Defense-in-Depth Principle