I keep thinking stakes in the ground to form a barricade or derail chariots or launch something like a catapult but not getting very far yet. A palisade makes no sense because that would be linear. Of course, multitasking is keeping me marginally attentive. (And I keep remembering that if we identify what the holes are for, we'll still have to know where they are!)

Haven't explored Hadrian's Wall. Staring at my office wall right now.