If you need a defense line, a good few planks shallowly buried with 6inch screws protruding at different angles around your perimeter! There are also battery operated alarms that you can string up knee height to fishing cord, when tripped they will alert you to and probably give any intruder a big enough fright that they might choose to go somewhere else first.
I've been thinking about this kind of thing for a long time, I lived in a pretty tough neighborhood for a while.