Accompanying article is here: https://arstechnica.com/gaming/2020/03/war-stories-how-princ...
Which links to the transcript: https://cdn.arstechnica.net/wp-content/uploads/2020/03/princ...
And his journals from the time, which are now being published: http://jordanmechner.com/journals
TL;DW: Memory constrains prevented the inclusion of enemies in the game (due to the space taken by rotoscoped images), so he got the idea of xor-ing frame pixels together to get the outline of the main character, kind of a shadow, and made that the enemy. He also squeezed a bit more memory from an "auxiliary" memory unit to animate guards.
Kind of light on details, though :)
If you want to delve into the details then the Apple II source code is on github: https://github.com/jmechner/Prince-of-Persia-Apple-II
Along with technical documentation: http://jordanmechner.com/downloads/popsource.pdf