Here are some cheats and tips

Grandpa
At the title screen, press L, Right + B, Right, B, R, Left, Down, A. Note: Do not enter the code too quickly.

Kaibaman
At the title screen, press Down, B, Left, Right(2), R, Up + R, A. Note: Do not enter the code too quickly.

Yami Bakura
At the title screen, press L, Down, Right, Left + B, R, Down(2), A. Note: Do not enter the code too quickly.

Yami Yugi
At the title screen, press R, Left, B, Right, Up + R, Right, Down, A. Note: Do not enter the code too quickly.

Chicago map
Win a game with Dark Magician Girl to unlock the Chicago map.

Domino Pier map
Win a game with Joey to unlock the Domino Pier map.

Kaiba Land map
Win a game with Mokuba to unlock the Kaiba Land map.

Ruins map
Win a game with Rebecca to unlock the Ruins map.

Space World map
Win a game with Kaiba to unlock the Space World map.

Toonworld map
Win a game with Pegasus to unlock the Toonworld map.

Scroll text faster
To make the text go faster, hold B.