There are retaining walls behind the Weingartens' house, under the old oak tree. But they are very carefully-laid walls. Ones with no mortar. No cement. Nothing holding them together at all except care and gravity.

Dry walls have been used for centuries, throughout humankind. Often, they consist of carefully laid stones, each selected for some certain property that only the craftsman sees, but that results in a solid wall.

These stones are uniform, but the principle remains the same.

By the way, gravity also makes the heating-cooling system work.