Head over Heels is notable because when your Sprite (mostly "Head" who could glide in the air) was in "altitude" you may not easily see were it is actually in the X,Z plane.
Yet putting a vertical shadow was may be heavier on the CPU and Ram consuming...
Example : It was used in Batman : the Movie (OCEAN) in the Batwing level (4th level, horizontal oblique) so you can see your altitude.
==Level design==