Rock always remains our first choice as a material for retaining walls. We prefer drystack retaining walls, that is, without mortar, although we certainly do our fair share of mortared walls. We also does a rock wall as seen above.
Other materials that can be used to make excellent retaining walls are pressure-treated wood or redwood, concrete, and concrete with a flagstone facing. Landscape walls should not exceed 3’ in height to avoid engineering and permits.