Three credits
Continue the game more than 15 times to increase the credits to 3 under the options menu.

Four credits
Continue the game more than 30 times to increase the credits to 4 under the options menu.

Infinite credits
Continue the game more than 100 times to increase the credits to "Infinite" under the options menu.

Extra lives
Lose the game with all lives gone. Continue to do this and you will increase the lives between 4-6.

Extra time
Lose the game with timer reaching zero. Continue to do this and you will increase the time between 70-80.

Time attack boss mode
After you defeat the final Boss by shooting him three times, he will hang from the tower. Shoot the chain and complete the game that way to unlock time attack boss mode.

Exchange a half of life for 5 seconds
Pause the game and press Up(2), Down(2), Left, Right, Left, Right, X, Circle. If you entered the code correctly, you will hear the sound of a gun.

Exchange 5 seconds for half of life
Pause the game and press Circle, X, Right, Left, Right, Left, Down(2), Up(2). If you entered the code correctly, you will hear the sound of a gun.