They don't step over things. They have to walk around a log, for example. You can keep them out with a single pick ax deep moat, theoretically, if it's done cleanly enough. (I had one pacing back and forth along a shallow river bank before.) It's just easier to make certain that you've broken the pathing, and haven't accidentally left any passages, with taller earth walls.

Personally, I try to make it high enough that I have to sprint to jump out, which does work out to about two characters high. It also helps to align with the compass for straighter walls. The more creases in it, the more likely there's a path there. The greys will be happy to show you if you've missed any spots.

They do reach over things though. Leave a wide margin between the moat and your builds or your garden to protect from attacks.