The question that everyone always asks is why is the ditch on the inside? It cant be defensive so must be a sacred site, it is the riddle of the ages?. The answer to this riddle is maybe this. There are two sorts of walls, barrier. That being manned and unmanned. If the barrier is manned like a pallisade then it would require a lot of resources in maintance and to defend them, but it does make sense to have an exterior ditch, to slow down the enemy whilst shooting him down. If an unmanned barrier like a hedge (and there is plenty of evidence for anchient boundary hedges) is to be used, as easy to maintain and does not need large numbers to defend. Then you have the central area with the standing stones for cover as you shoot the enemy trying to break through the hedge and negotiate the ditch. If this theory is applied to the Wansdyke you have outer defenses for avebury as well.