Can it climb a fence of cactus?
Alternatively, an overhang on the outer edge of the wall might keep them out?
Yeah, that's what I'm going to try, which is annoying as I need that cobblestone to build a road to my new penthouse on the other side of the island. If that doesn't work I'm going to use Netherstone to have a fire-moat around the walls. It's really annoying in my monster trap though, last night I went there to collect some bones from skeletons and two spiders stayed up on the roof of the channel so as to avoid the lava. 10 minutes of hissing.
Has anyone else found that their trees have gone horrible and diseased-looking? No matter what biome I go to, all the trees have predominantly dark leaves with patches of brown and green leaf blocks. It looks horrible. But, oddly enough, the trees that I planted in my fortress and underground tree farm are the normal vibrant green they always were, it's only the trees outside the walls that have gone weird.
If you're desperately short on cobblestone and don't want to spend the time mining the stone, you could always try the old column of lava & water trick. Alternatively, if you have enough lava and buckets you could always just line your outer wall with lava and a little moat at the base to make sure it doesn't spread. Aside from looking insane it'd make one hell of a light convenient light source.