Core wood beam works great. I've used that most often. I have tried moat, which really doesn't have to be any longer than a good long jump width and steep like everyone said above me. I found though that with that, occasionally they still pelt my fences with rocks. Bees at the corners tend to be good protection too. Next I plan on using both moat and earth wall up a bit higher so that the rocks can't hit so easy.

I'm not far enough along for flying critters yet, but I'm practicing some scaffolding building so maybe by the time I need to build anything in flying areas I can cover the whole place.