The codes for all systems to get the secret 3D first level of the original Prince Of Persia are:
(All used once a new game is started and you stay on the balcony - do not go inside.)
PlayStation 2 (PS2): (Holding L3 - pushing in the left analog stick) X, Square, Triangle, O, Triangle, X, Square, O
X-Box (XB): (Push the left analog stick in) Press A, X, Y, B, Y, A, X, B.
GameCube (GCN): Insert a controller into port four and port one. Hold B on controller four then quickly press A, B, Y, X, Y, A, B, X on controller one.
PC: Hold X and quickly press Space, Left Mouse Button, E, C, E, Space, Left Mouse Button, C.
There is a code you can input upon starting a new game (while staying outside on the balcony) that will load a 3D version of the first level from the original Prince Of Persia game from 1989. While tricky to do (it may take a few tries), and while the level/mini-game has no end (the team didn't have enough time to complete it), it still has two main goals (attaining a sword and getting into the secret room) and it's fun to go and see. Inside the secret room you will find 24 pack cases and empty bottles of POP drinks (a play on the European nickname for soda/soft drink - "pop" as in "bottles of pop" and the Prince Of Persia acronym as well), a giant poster of the POP Team themselves and stands full of Penguin Caffeinated Mints (which explains why the team put the company's credit into the end game Credits). The only sign of what the Penguin stand is, is because the stand logo has one word: "Penguin", a penguin logo and the mainly orange packaging. The company's website looks almost the same four years after the game was made. You can also use the Landscape button to see glimpses into the secret room, and it changes every time you swap screens.
This was built on a more powerful Jade engine, the same one used for Beyond Good & Evil.