Challenge mode levels
Press Down, Up, Left, Right two times, Left at the title screen. A sound will confirm correct code entry.
Expert and Impossible difficulties
Successfully complete Chapter 10 "Secrets And Salvation" to unlock the Expert and Impossible difficulty setting in Story mode.
Switching weapons
Normally you cannot change weapons except by picking up new ones. To access the weapon select screen, quit game play to the main menu then resume the level. Note: You will only have the default amount of ammunition for the selected weapon and will lose your previous stock. You can only choose one weapon, and will lose the other two special weapons.
Challenge mode levels...