1. Is there adequate drainage? If water flows down the hill, will it move away (or past) the wall or will it pool above the wall?
2. Is there a drain installed at the base of the wall? If a large amount of earth is being held back and the wall is tall, chances are you’re building a dam unless you have a drain along the footing.
3. How will the wall be laterally supported? There are various methods for supporting a retaining wall; geotextile fabric can be used, as can earth anchors and good old fashioned rebar.
4. What are you building the wall out of? Will it stand up to time? Timbers look great but will rot, concrete will last but can look unappealing.
At the end of the day, don’t build a retaining wall without talking to an expert. If the expert doesn’t talk about each of these options, they’re not an expert.