The Guardian Legend

Sound Test
Hold A + B on controller 1 and hit Reset to get the game's Sound Test.

Corridor Rush Mode
Enter the password "TGL" to access a bonus mode where you earn powerups by playing through all the scrolling shooter stages. For the Japanese version, use the password, "えいえるじい" ("ALG").

Message Debug
At any message panel, hold Select and press B to progress through all of the messages for the message panels, in the game's internal order, including 'unused' messages. This is possibly a debug routine that was accidentally left enabled within the game. Note that the game may glitch if you try to cycle too far...