Castle of Dragon

While walking through the corridor to the princess after beating the last boss of the game,, press and hold Up + Left + A + B + Select + Start on Controller 1. Something noticeable will change when you meet the princess compared to normal.

At title screen press, A, A, Left, Left, A, A, Left, Left, A, A, Left, Left, A, A, Left, Left, A, A, Left, Left, A, A, Left, Left, A, A, Left, Left, A, A, Left and Left. If done right, the debug menu should appear.