On flat ground a walled back garden is genuinely private: nobody can see over the wall and the wall is the whole story. On a terraced hillside it is not. The street one level up looks down into it, along with the driveways and front windows of every house on that street. Your back garden, your pool, your outbuilding and the doors onto your patio are all visible to a road you never think about.
That cuts both ways and it is worth understanding properly. It is protective in the sense that somebody in your garden is potentially visible to a whole street. It is unhelpful in the sense that what you keep back there — equipment, tools, bicycles, a well-stocked braai area, an obviously expensive outdoor setup — is on display to passing traffic that has no particular reason to be looking, and that you cannot see either.
The practical response is not to build a higher wall, which would achieve nothing against a downward view. It is to make sure the things visible from above are actually secured: the outbuilding, the pool store, the patio doors and the side gate. That is ordinary security lock upgrade work aimed at a specific and unusual sightline.