Security in depth layers independent controls so that no single failure grants access. A site might combine a fence, locks, credential readers, cameras, alarms and staff procedures, with each layer covering a weakness the others leave open. No single barrier is trusted to stop everything on its own.
One strong control is a single point of failure. Depth means an intruder who defeats the perimeter still meets detection, delay and a response that someone has thought through.
The layers in practice
Deterrence discourages, detection reports, delay slows and response acts. A fence without detection only relocates the entry point, and detection without a response produces nothing but a written record.
Layers must not share a weakness
Cameras, readers and alarms on one switch, one power supply and one internet link fail together. Independent power, separate network paths and documented procedures keep one incident from removing all of the coverage.