Press A, B, Right, Left, Right, Left, Down, Down, Up, Up at the title screen.
The game has multiple hidden messages from the developers.
For the first one, activate the invincibility code (press Up, A, Right, B, Down, A, Left, B, Up, Select at the title screen), but before the title screen goes away press Select, Up, B, Left, A, Down, B, Right, A, Up for a developer credit.
At the title screen, enter the Konami code (Up, Up, Down, Down, Left, Right, Left, Right, B, A, Start).
After beating the game, hold Select before the credits start for these pictures representing the staff.
Present at 0x13CB7 is a message similar to the Konami one above. This one references Namco, but there is no known button code to display it.
I AM NOT NANCO