I really like the thorny bushes, black and raspberries give some protection and produce fruit at the same time. I had some natural blackberries in a spot where I didn't want them, I've mowed then down forty times at least and the tough little buggers are still sprouting up! The hardest part of real protection is any normal house is not defensible, period. A 22 can penetrate any standard wall, and windows won't even slow it down. The best plan is to have safe areas that you can reinforce, and can have a way to return fire from. Also consider an emergency retreat path in case you're outnumbered. I haven't done it yet but am considering a concrete room in the crawlspace. Since your property is so large it may be a good idea to make a fortified store room in an outbuilding also, in case you couldn't make it back to the main house in a pinch. I think a 12" block wall filled with concrete will stop a 50 cal. If you have some peep holes for return fire you have a pretty good advantage. Hope you'll never need to test these ideas out!