At the 'Map Selection' screen enter the following codes to unlock the corresponding effect.
Unlock 2-Player Versus Mode: Hold L1 + L2 + Square and press X.
Unlock Xenon Unit: Hold L1 + L2 + Circle and press X.
Unlock Main Campaign and Original Campaign Levels At the 'Password' screen enter the password 'BATTLE' without the quotes to unlock 'Combat Simulation' mode. Then enter the following passwords to unlock the correponding level and Bonus. Main Campaign