The principle of Defense in Depth (DiD) is to provide multiple mitigations or protection mechanisms so that if one mitigation mechanism fails, there are additional protections in place to help protect assets from becoming vulnerable. Defense in depth protections are simply security best practices and are not triaged or rated by criticality. In many cases, good DiD mitigations can be more successful in offering protection against exploits than single point of defense mitigations. This is especially true when considering design or architectural DiD mitigations:
