I've posted quite a lot, about our tire-bale bermed house. Another application for tire bales is a retaining wall. We love the way ours turned out. The steps are all stone, with silver quartz flagstone on top, with stained concrete in the gaps.
We used 1/2 tire bales for this retaining wall. We wrapped wire around large rocks, and attached the other end to the bale cables, to use as "deadmen". Once back-filled, the rocks prevent the wall from ever moving forward. It took a day to stack all these 1/2 bales, tie-off the deadmen, and backfill. I had a friend, a retired large equipment mover, position the bales with a skid steer, while I wired the deadmen, and he backfilled. This is just a small section of the wall, probably about 1/4 of the 50 foot wall.
The bales were filled in with plastic bottles and trays, to fill in the gaps. Then a coat of adobe/portland cement mixture (10:1) was applied, filling all gaps, and covering all tires. On top of that, we used about a 3/4" coat of concrete - the green bag of Quickcrete. I did all the mixing, and my wife did all the applications, with gloved hands (no trowel). She also constructed some chicken wire coils and other shapes to add some artistic touches to the design. My wife applied a different combination of stains. The two of us could adobe about 10 linear feet in a day in about 4 hours (we're not Spring chickens). We could concrete-coat about double that.