Use earth in tight, layered packs. Press each layer until it feels firm, then let it set. Add a few stone ribs around the perimeter; they give the walls a solid spine without extra weight. Keep a small drainage slot in the lowest part so water won’t pressurise the walls. The simplest way to stay strong is to let the ground do the work—no bells, no bolts, just packed earth, stone, and a bit of patience.