Retaining walls are integrated parts of the landscape, so the number one priority is to make sure they can do their jobs and keep each level in place. These walls are also usually meant to reduce the effects of erosion between layers, so they'll usually be made of materials that can withstand the wear and tear that comes with being exposed to the outdoor environment. In a humid climate, for instance, a wood retaining wall might not do very well, but a quality one made of stone will.