I'm going to assume Rabbits has gone to the Batcave, not having had a response in regard to aspect, materials and height of said retaining wall.  I know he mentioned a preference to grow from the top to drape downwards.

There have been some good suggestions for sunny aspects.  Although I'd be a bit reluctant with some due to root invasion of said wall foundation.

I'll take an alternate shady aspect.  Rock orchids, bromeliads and air plants, stag ferns, ferns. Hung from the structure in pots - even better if it is a gabion type wall where pockets of soil are available.

DEB