Some retaining-wall blocks have locking mechanisms that allow them to be built to about 4 feet tall without extra engineering. Because we used tumbled blocks with no locking mechanisms, we used a construction adhesive on alternate courses. Remember, with locking blocks, you can remove blocks and start over if you make a mistake. But adhesive permanently bonds blocks, so work carefully.
Retaining wall idea: simple, inexpensive, durable, non-toxic, and easy to assemble retaining wall constructed using hollow concrete blocks. Because the blocks have been laid on their side, sloped backwards toward the hillside, you can plant vegetation in the their hollows exposed to the front. As vegetation is established, this will become a living wall. Similar technique to fashion steps as well.