Building this fountain will probably require taking apart the dried laid wall and excavating the earth out, installing the pump line and then rebuilding the wall around the water supply to the copper output pipe(pump will be in bluestone basin, running pump line up to copper pipe. ) Obviously, I have alot more research to do, as I haven't figured out how to get the pump electricity wire out of the basin in a watertight and unobtrusive fashion, or to have the pump tubing mate with the large (2" diameter) copper pipe. I know how to solder, so I was thinking of soldering a connection made out of copper pipe from the back of the 2"output pipe down the rear of the retaining wall and into the bottom side of the bluestone basin to meet the pump tubing there. What I am concerned about is having to backfill the dirt in over either copper piping or tubing (methinks copper is the way to go) and then having to maintain it. How do you maintain a retaining wall fountain? Does the pipeworks behind the retaining wall need to be always accessible? Is there anything I am not thinking about? Thanks in advance for your thoughts.

Does the pipeworks behind the retaining wall need to be always accessible?
No, but it should be able to drain back to the pool when the pump shuts off so that it does not freeze in the winter.
Is there anything I am not thinking about?
Freezing will damage fountains if not drained, use a GFI plug for the electric (Ground Fault Interrupt).
For possible large fountain designs see
http://www.strombergarchitectural.com/