Veilchenblau could work. It is extremely hardy and nearly thornless. I have one that is growing up a maple tree. It has long, long flexible canes. From HMF: Mauve or purple blend. Moderate, green apple, lily of the valley fragrance. Average diameter 1.5". Small, semi-double (9-16 petals), in large clusters bloom form. Once-blooming spring or summer. Habit: Climbing, thornless (or almost). Glossy, light green foliage. Height of 10' to 20' (305 to 610 cm). Width of up to 12' (up to 365 cm). Growing: USDA zone 4b and warmer. Shade tolerant.

Well, there's a guy on here, Botann (sp?) who says his miniature ivy ended up fruiting and seeding and going crazy and reverting back to English Ivy so there may be something to this, but I wish the websites and nurseries would explain that. It's so hard for a newbie like me to get correct information. I've spent an insane amount of money on my yard and freak out when a tree dies. Not going to rip anything out! Seems with an array of sharp tools I can control these plants. Just 1/4 acre with the house taking up a lot of that area. Your wall is the type more and more frequently being used to terrace new housing developments and there must be many ppl looking for something hardy that will grow up these walls and look good without ridiculous amounts of babying and maintenance. I've bought several varieties of euonymus fortunei in the last 2 months and hope they'll do their climbing thing. Looking each groundcover up in Google Images gives one an idea of how they can look, but doesn't usually tell one how LONG it takes to achieve that picture-perfect growth. Looks like the vinca will need to be pruned out of other plants like heather it's overtaking. These are raised beds so pruning is very easy. CostCo had nice grass shears and they're snip snip easy to use. Also got a wheeled weeding bench so can zoom down a line with shears for edging. One thing about these garden forums -- posters seem to attack a lot of plants and trees as invasive. That makes me laugh because every day the 'wild' around here is extinguished, visibly, and replaced with dead things. If only the plants COULD win! Not a chance anymore unless avian flu pandemic wipes the virulent pest humanimal off the planet. If ppl were still into gardening and installing beautiful landscapes with sprinkler systems it might be different. When I moved to Portland in 1984 there were so many estates with lush green landscapes and ppl took pride in their yards. But now the outdoors is a "bother" and ppl no longer cognize beauty or God's magnificent glory in old-growth natural riparian forest or any other original undisturbed remnant of Eden.

Hate to sound dense, but what do you mean, under a retaining wall?

How does it matter? (Not snarky, just curious.) Anyway, it is a west-facing wall so about half a day of sun.