By adding the short walls, and taller plants (as well as the arbor), the designer has created a sense of privacy and enclosure. This is the effect, although much more private, that we could strive for in the water wall garden with the additional raised beds and retaining wall, if you want to go that way eventually.
great backyard with a firplace next to a lower table and chairs. also like the lighted arbor with the greens all on it. and the brick wall next to fireplace.