At the main menu highlight "Versus" and in order press and hold L, B, R, and then in orfer release B,R,L. If done correctly, a cheat menu will appear. Enter the following codes to unlock these cheats. If you entered the correct code, Godzilla will roar.

Here are some cheats:

12 Contnues in Adventrue Mode: 548319
Add A.I. Player to Melee: 154974
All Cities: 107504
All Cities and All Monsters: 863768
All gallery items: 962129