Physical security describes both measures that prevent or deter attackers from accessing a facility, resource, or information stored on physical media and guidance on how to design structures to resist various hostile acts. It can be as simple as a locked door or as elaborate as multiple layers of armed security guards and guardhouses.
In a well designed system, these features must complement each other. There are at least four layers of physical security:
Environmental design
Mechanical and electronic access control
Intrusion detection
Video monitoring
The goal is to convince potential attackers that the likely costs of attack exceed the value of making the attack.