– A retaining wall foundation is typically laid down once the excavation is complete. This compacted earth or sand layer will hold the wall up and prevent it from moving. – The wall is then built up with layers of concrete, one on top of the other. The height of the wall will be determined by the slope of the land on which it will be built. – Once the wall is complete, a coat of paint or sealant is applied to protect it from the elements.