Five fundamental security principles of Defense against attacks

Diversity

Closely related to layering, if you are using layers of security you must use different types of security for each layer. Attacker must use different techniques to attack each layer.

Simplicity

Making a security system harder to use may make the users lazy and create bypasses. Also if security is complex to set up then it creates more work for the security professional and they may miss a vulnerability.

Obscurity

Making it more difficult for an outsider to recognize what is going on inside.

Limiting

People should only be authorized to the information they need for doing a task. Access must be restricted to a minimum.

Layering

Provides the most comprehensive protection. Instead of one security defense you have multiple defenses. If one of the defenses is broke the attacker must then penetrate the next layer of defense. More layers add more security, but is limited to the vulnerability of each defense.