Unclaimed land is like leaving your stuff on the river's bank. Anyone and everyone will take it. I definitely would.
Claimed land is saying that you intend to keep it, but you're not prepared to defend it. Normally I wouldn't mess with this, unless the person had poppy seeds or steel (I'm lacking poppy, alright?).
Claimed land with a roundpole fence will keep the baby griefers away. They need vandalism to get in, and that's a higher skill costing more LP. It'll help Mr. Random-Thief stay away, but if they have vandalism, then the fence will make no difference.
Claimed land with palisade makes you a real fort. You're invested in your land and it'll take some effort to wrest your equipment from you. As long as your gate is closed.
Land with a brickwall, claimed or no, is a proper fortress. No one is realistically going to mess with you once you get this. 75 soak means someone has to have 75*75 str to do any damage with their bare hands (that's 5,625 +1 str to do 1 damage at a time). Even with a ram and four people manning it, that's a lot of strength, and it also leaves them standing still for your archers to peg them
