This enables a debug menu which was obviously used by programmers during development but made unavailable with the final release build of the game.
GAME START starts the game as normal.
STAGE SELECT allows you to select your starting stage.
BG test is the aforementioned map editor. I don't understand it fully, but it lets you view only the background data for each level. The Pad moves a cursor around. Pressing I or II moves the cursor around more quickly. Holding Select+U/D/L/R alters the window/2nd background layer settings, sometimes masking the background. Pressing Select+I opens up the gate at the end of the level, but it also makes the gate appear at other points in the level. RUN exits the BG test.
Sound is a regular sound test, as far as I can tell.
SP test loads up a stage, but only shows the sprites in the stage. For some reason, only the sprites for stage 1 are shown. You can upgrade your armour (or change into a duck) by pressing RUN, and you can take away your armour by pressing Select, which also changes your current weapon.
ENDING displays the ending montage (minus the music).
OPTION takes you to the regular options menu available in the game.
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