Easy Concrete Bag Retaining Wall. Stack up bags of concrete to form a wall. Hose them down with water. They harden and later the bags decompose leaving a super hard wall behind. No molds, no mixing, no pouring, no rentals, no mess! Don't like the look? You can cover it with a veneer later.
If you have a natural source of rocks (your property over run with rocks?) .... OUTDOOR | retaining wall fence I like the idea of having a wooden fence filled with cool looking rocks as a privacy fence in the backyard, but that would not be cheap.