Secret characters
Press A, R, L, A at the main menu when "Press Start" is flashing. The heads of two secret characters will appear at the bottom of the screen to confirm correct code entry. You can the select the two secret characters in any mode.
Another world
Press B, R, L, B at the main menu when "Press Start" is flashing. A planet will appear at the bottom of the screen to confirm correct code entry. Select puzzle mode to access new levels.