You can see here that each row of block will set back from the front of the lower level. This ensures the wall won't tip forward and fall. I learned today that these walls can be built safely up to a height of 4 feet, and was told that one can go even higher so long as the wall doesn't need to secure a large load behind it. Recall that most retaining walls are serving a functional purpose of holding back a span of dirt on a slope, which can create great pressure against the wall. In our application, the wall will have very little to hold up (about 18 inches of soil and aggregate) which will be secured on the other side by the Stupa itself. This means the wall doesn't need to have as much structural strength as one that would be used to hold a landscape against errosion. That's good news, in that we can feel free to build as high as we see fit...