At the title screen, press Down, Up, Left, Right two times, Left to unlock four Challenge mode levels. If you entered the code correctly, you will hear a sound.Expert and Impossible modes
Successfully complete Chapter 10: "Secrets And Salvation" to unlock the Expert and Impossible difficulty.Switching weapons
Normally you cannot change weapons except by picking up new ones. To access the weapon selection screen, quit the game, exit to the main menu, then resume the level. Note: You will only have the default amount of ammunition for the selected weapon and lose your previous stock. You can only choose one weapon, and will lose the other two special weapons.Hidden message
Take the first letter of each chapter and arrange them in order to spell out the sentence "Warren Lies".