In Site Security Planning and Design Criteria, security expert witness Randall Atlas Ph.D., AIA and Anthony DiGreggario of Atlas Safety & Security Design, Inc. write on security layering: the Onion Philosophy:
The first layer is the outside skin of the onion which translates to the site perimeter of the property. The building skin of the architecture is the next layer. Sensitive areas within a building are deeper layers requiring protection, and finally special persons,
information, or property may require point protection or the center of the onion. The site perimeter is the first, not last, line of defense. The State Department seeks setbacks of at least 100 feet for new buildings and even that distance is difficult to obtain in most urban settings. While most perimeter fences and walls are designed to discourage intruders, they are of little use against a determined person or bomb vehicle.
Designs are now available for vehicle-stopping capabilities. The building skin is the next layer of protection. It is possible but never easy or inexpensive to minimize openings,
orient them away from the perimeter, raise them above the ground, and provide windows, doors, grilles, and other devices that resist ballistic weapons, explosives, and forced entry all the while trying to retain a sense of openness, operable sashes,
unobstructed views, and adequate natural lighting. The next layer is the interior space planning security. The most sensitive areas should be located high and away from exterior zones. Thought must be given to the use of spaces behind or near windows.