In POP II there's an hidden reference to the second episode of the 2D serie, Prince of Persia: The shadow and the flame.

To find it, play the game until the Prince, transformed in the Sand Ghost, is running around the caves beneath the Time Fortress. Right before exiting this area, you may fall down into a large hole. When you're there, look on the floor for a switch hidden by the mist. Press it and you'll open a hidden door right behind you. Enter the new room and break the weapons rack on the left: you'll find the Flame Sword from the original game.