Here we're talking about retaining walls made from either cinder-block or a formed poured concrete wall to which a veneer stone is applied. These types of walls require a concrete footer dug at least the frost line which in our area is 18". Depending on the topology and height of the wall additional reinforcement (internal rebar and concrete poured in the cinder-block cores (or both) may be required. NOTE: Either way if the wall is 4' or higher a building permit is required along with a certified footer plan from a professional engineer. That footer plan will also have to be approved by the building inspector prior to a permit being issued.