Cladding/Beautification of wall in the garden

I guess by Mauerkrone you mean the top layer that protrudes a bit, right? That was cast like that back then, but I wanted to remove the protruding part with a big angle grinder.

It was cast like that back then, but I wanted to cut off the part that sticks out with a big angle grinder.

Man is the architect of his own misfortune - I would consider that exemplary foolishness worthy of Schilda. The crown at least protects your construction adhesive against water running directly from above, doesn’t it? :-(

Hmm.... of course it depends on what you want or what effort you want to put in for it. For me, the effort to work away a 12m strip of concrete etc. would definitely be too much, hence my idea.
You take battens or 6x4 timbers or something of that thickness as a substructure, so that the later cladding with rhombus slats (or battens/boards... depending on taste) will cover this ledge; then you won’t see it anymore because it disappears behind the cladding. Likewise, I might possibly lay wood on top here, for example either large, rough-sawn construction boards (possibly pressure-treated) or decking boards. Then you can sit on it and in my opinion it looks good.
If you want to cover this 12-meter-long wall with natural stone or similar, it will become very expensive unnecessarily and not necessarily more beautiful.
If a part turns out ugly, you replace it with 1-2 screws.
Of course, you could also take colored concrete slabs but these would then have to have the appropriate depth and usually do not look very nice from the front. Or you raise the wood slightly higher at the front and plant something nice behind it with inexpensive, wide planter boxes (cheap flower troughs, wide cement buckets from the hardware store etc.) which you wouldn’t see because of the raised cladding.
You can, as you say, then hang planter boxes as desired or screw them on anywhere you like.
For the substructure you could, for example, divide the wall and simply clad it vertically 3-4 times or screw rectangular planter boxes or metal brackets etc. directly to the wall, on which your plants stand.
I might even place higher troughs (very stylish, for example, also Corten steel) on the ground, so against the wall, and then run the cladding against them. Likewise, you could stack black planter stones/grid stones or similar and plant them... or put a fire bowl on top. There are no limits to the romance there.

My idea was to remove the current overhang and then place a stone slab or something similar on top. This would then protect the underlying cladding (if there are any).