Fences and walls. Fences, walls, and retaining walls shall be permitted, provided that no fence or freestanding wall shall exceed six feet in height, unless located in the front yard between a principal building and any street or roadway, in which case no fence or freestanding wall shall exceed four feet in height. Retaining walls may exceed six feet in height, where the approving board finds that the additional height in wall outweighs the amount of disturbance a tiered or multiple-tiered wall would require. Any residential building rear yard that faces a downsloping retaining wall shall provide a four-foot fence at the top of the retaining wall. The approving board may, in appropriate situations, require such walls to provide landscaping to be planted in front of or atop of such walls in order to reduce their visual impact. In no case shall a retaining wall exceed eight feet in height. For the purpose of applying height limits, multiple, staggered or tiered walls, and/or wall/fence combinations shall be considered single units unless the horizontal distance between the top of any section or tier shall meet or exceed five feet.