RETAINING WALLS

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So, I have these lush retaining walls in my back yard. They are multiple tonne sandstone blocks.

You can see that they have nice returns.

However, on the other side of the walls there are no returns. That's thanks to a builder who took off.
That side is where the walls say hello to a soon to be in place fence.

What's the best way to retain the side that meets the fence? Should I drop some steel bars into the sandstone foundation and frame up to make a concrete ending that I can semi bury??

I'll drop some images so we can think because I've been thinking too much.