Trick mode
Keep pressed L2 + R2 on the main menu for the code introduction screen to appear. Then introduce:
141421: All monsters and activated levels.271828: All levels activated.071767: Destruction of buildings with one blow.986960: Invincible to bullets, rockets and projectiles in general.011235: All monsters with power improvements.082864: Demo mode with random monsters.667300: See the intro of the game.667301: See the end of the game.667302: See the credits.