For the plains, I have a stone tube, cart width and iron gates at the ends. Never had anyone get through yet, also previously had wooden gates. Again no ingress.
In meadows, trolls just smashed at the unditched walls, ignored gates. In black forest, skeleton surprise stood near wall where boars were on other side, no ditch.

So I put a gate up, and often a ditch, remember to shut it, and to be honest not had wandering monsters. So my suggestion is... a gate!