I've never used it, but if I did decide to make my walls permanent, I would use "concrete glue" instead of mortar.
This product is supposed to be somewhat flexible. It comes in tubes like caulk, for a caulk gun. Squirt, squeeze together, done.
I tend to rely on the wall sloping inwards for stability, instead of thick retaining walls. I do very small-scale stuff! This is my biggest prject, done over three years and still not finsihed: really three beds on a slope, then a wlakway and stump, then two more raised beds.
Acrtually, this plus the sidewalk is most of my front yard!
from below, ------------------ also below, -------- from above , adjacent bed
distant shot -------------------- zoomed closer
