Invented by the Vietnamese, the Punji Pit is the simplest booby-trap there is.
About this creation
Step 1: Dig a hole & fill it with wooden stakes, rusty pipes, and other sharp objects. For added affect, add feces. It may seem gross, and it is, but it will cause infections in the survivors of the pits. This will lead to complications that will permanently take them out of the fight.
Step 2: Place sticks over the hole.
Step 3: Cover the sticks with leaves and such.
Optional: Put a little added incentive to make walking over it more likely.