I just read something on another blog that brought something to mind about perimeters I have been thinking for a long time.
I had a guy over a few months back who wanted to know why I had so many non-lethal but otherwise annoying hindrances prepared in my retreat defense. What is the purpose of it all if it doesn't kill/cripple an attacker immediately?
The answer is simple. Nobody loses a firefight with a guy who has one foot in tanglewire, another stuck in a beartrap, partially burned by a napalm blast who is staggering about screaming because he walked into a cloud of stinging chemical mist. Sure, he's not dead at that point but it's pretty easy by this time to change that very quickly. For him, strategy has come to an end. He's just reacting at that point and there is a limited range of reactions available to somebody in that predicament.
For this reason, every hindrance you can come up with that puts a trespasser in an awkward or ridiculous situation or makes him increasingly uncomfortable reduces all his options at that stage. He may not be quicklimed yet but it's really your discretion what to do with him once he is in that bad a predicament.
We studied this quite a bit in my psy-ops course in the military. It's the ancient maxim about how a war is lost for want of a nail to hold on a horseshoe. It doesn't take much at all to reduce a person's threat level to zero when they assault a fortified position. This is why militaries sacrifice thousands to overturn a simple fortification, by throwing people at it until it finally gives by sheer volume. The average person isn't interested in kamikaze attacks leading to certain death and doesn't know anybody who is interested in doing it on his behalf, either.