<P> The initial layer of security for a campus, building, office, or other physical space uses crime prevention through environmental design to deter threats . Some of the most common examples are also the most basic: warning signs or window stickers, fences, vehicle barriers, vehicle height - restrictors, restricted access points, security lighting and trenches . </P> <P> Physical barriers such as fences, walls, and vehicle barriers act as the outermost layer of security . They serve to prevent, or at least delay, attacks, and also act as a psychological deterrent by defining the perimeter of the facility and making intrusions seem more difficult . Tall fencing, topped with barbed wire, razor wire or metal spikes are often emplaced on the perimeter of a property, generally with some type of signage that warns people not to attempt to enter . However, in some facilities imposing perimeter walls / fencing will not be possible (e.g. an urban office building that is directly adjacent to public sidewalks) or it may be aesthetically unacceptable (e.g. surrounding a shopping center with tall fences topped with razor wire); in this case, the outer security perimeter will be defined as the walls / windows / doors of the structure itself . </P> <P> Barriers are typically designed to defeat defined threats . This is part of building codes as well as fire codes . Apart from external threats, there are internal threats of fire, smoke migration as well as sabotage . The National Building Code of Canada, as an example, indicates the need to defeat external explosions with the building envelope, where they are possible, such as where large electrical transformers are located close to a building . High - voltage transformer fire barriers can be examples of walls designed to simultaneously defeat fire, ballistics and fragmentation as a result of transformer ruptures, as well as incoming small weapons fire . Similarly, buildings may have internal barriers to defeat weapons as well as fire and heat . An example would be a counter at a police station or embassy, where the public may access a room but talk through security glass to employees in behind . If such a barrier aligns with a fire compartment as part of building code compliance, then multiple threats must be defeated simultaneously, which must be considered in the design . </P> <P> Another major form of deterrence that can be incorporated into the design of facilities is natural surveillance, whereby architects seek to build spaces that are more open and visible to security personnel and authorized users, so that intruders / attackers are unable to perform unauthorized activity without being seen . An example would be decreasing the amount of dense, tall vegetation in the landscaping so that attackers cannot conceal themselves within it, or placing critical resources in areas where intruders would have to cross over a wide, open space to reach them (making it likely that someone would notice them). </P>

This activity allows attackers physical access through secure doors