I'd like to clarify one point about my theory that the sloping side would be on the far side of the hole from the tower. The purpose is not to reduce the height of the tower (this is merely a consequence). The reason for having the slope is so that the block can start its descent into the hole while it is still rotating off the tower. When it makes contact with the near side of the hole (nearest the tower) and commences its rotation to vertical, it is already well located into the hole and merely has to drop to the bottom. Given that we need the block to be not too far from vertical as it enters the hole, any other arrangement involves the risk of the block missing one edge or other of the hole and/or impacting the edge of the hole with sufficient force to move earth. I'd like the block's rotation from the tower to be stopped by a good wall of earth of at least a foot or so.

It also makes sense to erect the stones from the outside of the circle, otherwise you have to bring at least some of the stones inside before erecting them.

Having said that, I would still like to know what the facts are. Surely there must be some documentation of the excavations that we can access.