A retaining wall holds back soil, but what actually pushes on it is water. Saturated soil weighs far more than dry soil, and after a week of November rain, the ground behind a wall with no drainage is trying very hard to fall over. Walls that lean, bulge, or tip after a few years almost always skipped this part.
The right build starts with a trench and a compacted gravel footing, so the first course sits level and doesn't settle. As the wall goes up, each course steps back slightly (called batter), which lets the wall lean into the slope instead of away from it.
Behind the wall goes clean drain rock, wrapped in fabric to keep soil out, with a perforated pipe at the bottom that daylights somewhere downhill. That pipe is what lets water leave instead of building pressure. Then the soil goes back, and the lawn or beds go on top.
Walls over four feet usually need engineering and a permit in most of our service area, and we'll tell you up front if yours does. For the typical two- to three-foot garden wall or terrace, proper footing and drainage are what keep it standing. It's a few extra hours of work that saves you a rebuild.