The first piece of a physical protection system that comes into play, whether in deterring an intruder or attempting to stop the attempt at intrusion, is the perimeter barrier. This barrier can take many forms, from a fence or wall meant to physically obstruct entry, to signs that deter trespassing by ensuring property boundaries are known.
As with all elements of security, the best solution will vary based on the environment and the purpose of the property. In some cases the threat level is low enough that simply marking the area the owner does not wish to be crossed is enough to stop intrusion, or more there could be a worry that more oppressive measures could ward off potential customers.
In other cases, the threat level is high enough that more defenses are needed. To an extremely motivated intruder, a fence or wall will only delay their entry. However, as one of many delay barriers to be overcome, in addition to the ability to limit access points to certain areas, perimeter barriers remain a significant element in both deterring and stopping intrusion.