Inspired by mythbusters. Before you start flaming, note that I'm talking about a period security system.
Make a drum out of dried skins and wood, bury (well, half-bury) it on your property. Vibrations in the drum are felt when people are walking (and especially digging, woodcutting, etc.) in the general area. It should be an audible sound when fairly close to the drum. Range at which the origin can be before you hear it depends on your perception and the quality of the drum.