If you are looking for perennials that would grow near the top of a rock wall, and I am going to assume they would get good drainage and have at least half a day of sun then I suggest Sedum 'Autumn Joy' for fall color, Salvias for summer color and Euphorbias for spring color. Mix in some lower growing perennials such as pulmonaria, coreopsis and creeping sedum 'Angelina' and you'll have a good start at a perennial display that is drought resistant but full of color and different textures. Keep growing, Marianne Binetti