Pure speculation - a typical siege strategy was to dig under walls to cause them to collapse. First, the earthen mound would make the tunneling to collapse a much more labor intensive effort. Second, if an enemy was at the base of the wall it could actually be easier to hit them with projectiles at this angle rather than leaning over and aiming straight down.
Again, I have zero evidence to support these points, just spitballing here.
The Ark of Bukhara is a spectacular-looking fortress located in Uzbekistan, built 1,500 years ago (lemmy.world)