"Security-In-Depth is a military concept that means placing a series of progressively difficult obstacles in the path of an aggressor. These obstacles are often referred to as 'lines of defense' or three rings of security.

The first line of defense is at the property line. Methods of defense at this point may be either natural, such as a river or man-made, such as buildings, walls, fences, signage, etc. At a very minimum, the property boundary must be defined in some way so it separates itself from its neighbors. Lighting can also be present.

The second line of defense is the exterior of buildings. Controls at this point should be difficult to overcome. It is important to remember that all six sides of structures, (roof, floor and walls) often present weaknesses that must be strengthened. Special attention must be given to unusual points of break and entry: doors, windows and skylights.

The third line of defense is interior controls or object protection. Controls at this line of defense include electronic motion and intrusion detection devices, access controls, safes, vaults, document storage cabinets, quality locking devices, and fire protection.

Barriers serve basic purposes: physically deter/delay, psychologically deter/delay and/or complement security personnel needs. Positive barriers should be established for controlling vehicle and pedestrian traffic flow, checking identification of personnel entering or departing, and defining a buffer zone for more highly classified areas. When the greatest degree of security is essential, two lines of structural barriers should be installed on the perimeter. These barriers should be separated by not less than 15 feet and not more than 150 feet for best protection.

Doorways, including the frame, jambs and stops, are constructed of either wood or metal. There are two general uses of doorways: personnel and vehicular. Regardless of the design or the size, doors have weaknesses. A door is often much weaker than the surface into which it is set. The door frame may also be a weak spot. If not properly installed, hinges may also contribute to the weakness of a door; if the hinge pin is easy to access, it may be pried off. If the mounting screws are exposed, they can be removed.

Windows are designed to provide ventilation, natural illumination, or visual access through a wall, or any combination of the three. Windows, in areas where it is necessary to deny visual access, are glazed with frosted, pebbled, or other opaque or translucent glass. An intruder can easily cut out a section with a glass cutter, or it can be broken. If they are not strengthened, standard windows may be the weakest link in the barrier protection of a structure.

The most widely used method of controlling physical access is the lock. Locks are used on doors, vehicles, safes, in desks, briefcases, display cases, etc. Locks are also one of the oldest of security devices in use today.