Alarms: Take as much 10 lb. test fishing line that will surround your BOL and tie it to a bell of some sort that way if someone steps on or trips over the line it will ring the the bell. Also you could invest in motion detector flood lights and post them strategically around your property.
Booby Traps: I know it is illegal to trap someone who is trespassing but after a TEOTWAWKI you are going to need all of the security that is available. One really simple one is is take a shot gun shell and a mouse trap. take the trap and drill a hole that is big enough for the shotgun shell to fit through and there you go you can use your imagination from there. Another is a leg hold trap. Dig a hole around 2 feet deep and drive steaks into the side of the hole angled down all the way around the cylindrical hole.
Other alarms: I would suggest a well trained dog would be an excellent alarm and defence.
