How can something walk around earthen walls? They should totally enclose your base, if not, you need either a ditch or finish the wall, or both. Or a cliff will work if it is high enough. Glad you found the culprit. Now you know the drill, also maybe every so often check the coverage, especially after troll raids as their log damage goes in about 3 floor squares deep. Veggies and trees don't take damage but buildings do, anything made from stone or wood. Earth walls are impervious.