In the event a fence is placed upon a wall, the total height of said wall and the fence, collectively shall not exceed the height limits prescribed herein. If, however, a fence is placed upon a retaining wall, then the wall, individually, and the fence, individually, shall not exceed the height limits as prescribed in this subsection. Shrubbery and bushes shall not be permitted on a retaining wall unless a fence of height equal to the shrubbery or bushes shall be placed on the retaining wall in accordance with the height restrictions as set forth in this subsection;