After you beat the game wait for the credits to roll. There will be an extra level called Cake Walk.
Cheat Mode
To enable cheat mode, pause the game while firing and press B, A, B, A, A, A, B, A, B, A, C, X. To make use of your new-found power, enter any of the codes below:
SKIP MISSION: B, A, C, C, A, A, A, X
INCREASE MISSION NUMBER: C, A, A, A, A, C, A, X
SUPER MISSILES: C, A, A, B, A, X
SUPER LASERS: C, A, A, B, A, C, A, X
INVINCIBILITY: A, B, A, C, A, A, B, A, X
SMART BOMB (HITS ENEMIES AROUND YOU): A, C, A, B, A, A, C, A, A, A, X
REFILL WEAPONS: B, A, A, A, A, B, A, X
DISPLAY PLAYER'S NAME: B, A, B, X
DISPLAY PROGRAMMERS MESSAGE: B, A, C, A, C, A, X