Three extra continues:
Press Down/Left + 1 at the "Game Over" screen.
Continue game play:
Press Up(2), Down(2), Left, Right, Left, Right, Down, Up, Down, Up at the "Game Over" screen to continue up to nine times.
Press Right, Left, Down, Up at the title screen.
Enable the "Sound test" code. Play sounds 7, 4, 3, 7, 4, 8, and 1, pressing 1 after each sound. Options to play the game as a jet, switch the vertical controls, and adjust the difficulty level will be available.
Hold Left at the title screen to view the game's last Boss, Haya-Oh.
Enter ERI as initials in all seven slots at the high score screen.
Hint: Bonus points:
Reach the round 5 bonus stage. Try to hit less than three trees and weeds and do not try to get points. At the end of the stage you will jump off the dragon and a plane will fly across the screen. Shoot it down to get 10,000,000 points.