Treated wood provides the least expensive solution for retaining walls and walks, but it also the least durable. Expect to replace wood retaining walls and walkways in 15-20 years. The drawback to wood is that you are generally limited to straight walls and edges. This can be somewhat restrictive in the landscape design and sense of flow. But where the budget is the overriding concern, treated wood can provide a quick and least-cost solution and provides a natural textured look and feel.