A retaining wall is not really holding up soil. It is holding back the water in the soil. Dry earth mostly stands on its own at a reasonable angle; it is saturated earth, which is far heavier and behaves more like a liquid, that pushes walls over. Almost every failed wall we get called to look at failed for that reason.
So the two things that matter most are both invisible once the wall is finished. The first is the footing: wide enough and deep enough to sit on undisturbed ground, not on the loose fill that was pushed around when the lot was graded. On the sloped lots common through La Cañada Flintridge, Altadena and the upper streets of Monrovia, that often means digging deeper than people expect.
The second is drainage. Gravel backfill and a perforated pipe behind the wall give water somewhere to go, and weep holes let it out at the face. Without them, every rain loads the back of the wall with water pressure it was never designed to take. A wall with beautiful stonework and no drainage will lean; a plain block wall with proper drainage will not.
Above that, it is about detail: courses laid level, the wall stepped correctly on a slope, and the cap and the steps tied in so the whole thing reads as one piece of work rather than three.
Planning a wall, or worried about one that is starting to lean? Call (323) 243-7505.
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