retaining walls are usually under 10-foot high if they are designed based on gravity with regular toe and heel foundation. These walls are treated on the backside with a waterproofing chemical or a plastic membrane which is backfilled with gravel to let the water run down to a 4-inch diameter PVC pipe perforated on top every 6-inches installed with a slope of 2% to 3% on the bottom of the wall draining out into storm drain system. There is weep holes as a back up every 4-feet by either 1-inch PVC or ornate cast iron pipes.
Retaining wall shorter than 8-feet are usually made by concrete blocks and the same general detail of drainage applies to them except the weep holes are created by leaving every 4th joint on the first row ungrouted.
Taller retaining walls or the new thin retaining walls anchored by horizontal epoxy coated tension cables (nails) have elaborate drainage systems engineered by the geotechnical soils engineer as an integral part of the backfill drainage network.