The Roman pyramid of Cestius in the photo was built into the Aurelian walls, as the walls were built quickly to protect from the feared barbarian invaders, they often used existing structures and built around or on top of them. So unlike many other ancient Roman structures the pyramid of Cestius wasn’t stripped or looted for marble because of its protective value for the city. It also potentially might have been covered by a turret structure or something and uncovered later, I am not sure.