Retainer walls are built from treated 6×6 timbers man or made wall blocks, or natural choprock. Wooden retainer walls are typically set directly on the ground after grass removal and excavating to level, a granite gravel base can also be added for stability, then rebar first course to the ground and spike additional courses together. Landscape blocks are placed on either a granite base or a concrete footer for additional stability, then using block glue, gluing additional layers in place.
Natural stone walls are always on a concrete footer then all courses are mortar set together, height and lengths of wall based on elevation and specific needs.