Several comments:
I've always wanted to have some signs printed up saying "Trespassers will be eaten."
On the serious side:
In many (most) areas of the U.S., the Castle Doctrine only applies to the inside of your house/trailer/car, not all of your property.
In many areas, you can legally shoot someone trespassing on your property, but ONLY if you have very specifically worded signs at minimum intervals around your property. Using your creatively worded signs won't cut it legally.
I knew some of the comments were in jest, but for anyone who might not know, in most of the U.S. at least any kind of intentional booby trap is illegal even if no one has any legal reason to be there. And they are certainly the definition of "premeditation" if that turns out to be important.