So my problem is that the inner garden nearest the tower is described as being raised above the normal city level.
So sort of like a tiered cake?
In this case, given the walls are roughly the same height (Conan can leap up and grab the coping) it makes no sense that the ground on the inner wall is higher than the ground on the outer wall, but the walls are the same height.
The only way I can reconcile this is that the ground on the outer garden slopes upward toward the raised higher inner garden.
Sort of like a motte and bailey.