Talking with people about succulent care or observing succulent care "in the wild" has clearly demonstrated to me some of the confusion that exists throughout the world of horticulture with regard to succulent plants. Just take a walk through garden centers where highly trained nursery people are on staff. Beautifully maintained bedding plants, fruit trees, and a wide variety of ornamental plants abound, perfectly watered, fertilized, and maintained. Then seek out the succulent section. What you'll see are mislabeled, over watered, under watered and in general poorly maintained plants. I puzzled over this for many years as I was asked to help landscape professionals and retailers.
In order to garden successfully with plants growing on a wall, it is important to recognize the difference from traditional gardening. I never thought about gardening on the ground or in pots as gardening horizontally, but that is exactly what you are doing. Of course there are variations in the slope, but by and large, traditional gardening has been horizontal. Succulent plants are often found growing on cliffs in the crevices of rocks or on the cliff edges, clinging to what soil they can find purchase in. This ability to eke out a living in little soil makes them pretty good candidates for vertical gardening.