I think that would become a safety issue as the soil would make it very heavy and no, you couldn't grow pyracantha in it, that's a large shrub with big roots.
Spiky plants are the way to go - climbing or rambling roses that repeat flower and can be trained on wires stretched horizontally across the wall and lopped and arched at the top. Pyracantha is an excellent plant of spikes and wildlife - shelter, flowers for nectar, berries for birds.
Vendée - 20kms from Atlantic coast.
"The price good men (and women) pay for indifference to public affairs is to be ruled by evil men (and women)."
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I'm guessing you're suggesting fixing a ledge/shelf alongside the inner wall, which'd mean the plant evenually protrudes above the height of the wall?
Ah sorry, I suppose you mean a 2' to 3' _depth_ hedge