I took a video of a retaining wall that is starting to lean over. The Pisa system used larger stones at the bottom of the wall and the top would use narrower stones. Upon inspection it looks like right where the change over to smaller stones is where the wall is leaning. As you can see in the cross section. Larger stones on the bottom and than they reduce the width as the wall goes more vertical. As I say in the video the base is in good shape and looks like it hasn't really shifed too much. Stay tuned for more retaining walls in various states of leaning, or as we say in the industry, "rotation" . My guess is that if this wall was re-designed they would something like Siena stone. A stone with more weight to hold back the slope.