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.
Add cap blocks to the top row of a block or stone retaining wall. You can add a layer of masonry adhesive to hold them in place, then back fill. Or you can backfill as you go, then add the cap. Either way, it provides a nice finished look, much like edging added to a paver patio.