Tried to do some research beforehand. The frame of the box is built with treated lumber including joists running lengthwise and blocking in the middle. There are six supporting 4x4 treated posts. I used cedar slats on top of the joists, and then entire walls are built from cedar, including facia pieces over the 4x4s in the corners such that no treated wood is in direct soil contact. Landscape fabric lines the walls and sides. We live in the woods as you can see, so of the 18 inches of depth the lower 6 inches is sticks, pine cones, pine straw, dead leaves, and forest floor humus. Purchased vegetable gardening soil for the rest. It didnt collapse yet so Ill take that as a good sign.