At the "Press Start" screen, quickly press Up, Down(2), Right(3), Left(4), then hold Circle and press Start to unlock all the Bosses as well as the engineers and a few army characters in multi-player mode. If you entered the code correctly, you will hear the sound of a gunshot. Most of the Bosses have odd guns and may not be able to back up against walls or roll. Alternatively, successfully complete the game on the easy and normal difficulty settings. Note: The code must be entered before demonstration mode begins.