Install range markers on the trees up the hill so if you have to snipe, you have the ranges already laid out.
Determine the best spots that an attacker would use up on the hill and decomission/booby trap them (or ruin them with a wasps nest or a bucket of crap)
The hill is good for shelling you, and sniping from above, but eventually they will wanna come down the hill, d when they do you will be able to predict their general path. Mine it, modify it, or build your defensive plan around ambushing them when they do.
Down here we have something called cat claw, and some Mesquite trees specialize in thorns so big they will easily penetrate boots. Jumping Cactus works great for keeping people out of specific areas. You prolly have poison sumac n such.
Install and conceal one of those wireless gate sensors up in the trees. You will hear the chime whenever people are messing around up there.
Get a big, ugly pit bull from the pound. There are lotsa those.