Bonus levels
Type A, up, B, down, A, left, B, right on the title screen.
Extra Continues
Enter the Options menu and put the cursor next to the credits option. Press Left, Right, Left, Right, B, A, A, B, Right, Left, Right, Left. Press A as many times as possible in the 29 seconds after that: every 10 presses will yield an extra continue (up to 29 more).
