At the Main Menu, hold L, then B, then R. Now release B, then R, then L to bring up the code entry screen.
Now enter any of the following codes:
696924 Unlock nearly all monsters 202412 Unlock Orga
480148 Unlock all cities
661334 Technicolor mode 567980 Black and white mode 760611 11 lives 112122 Indestructible buildings 650867 No energy gain for Player one 443253 Take off life and energy bars 492877 Health regenerates 176542 View credits 097401 View version number 986875 P1 is small
971934 P2 is small 895636 P3 is small 795735 P4 is small 174204 All Players are small 843901 P1 is invincible 706149 P2 is invincible 188522 P3 is invincible 286552 P4 is invincible 505634 All players are invincible 511012 P1 deals quad damage 815480 P2 deals quad damage 212454 P3 deals quad damage 286552 P4 deals quad damage 817683 All players deal quad damage 649640 P1 Always in Rage 122224 P2 Always in Rage 548053 P3 Always in Rage 451242 P4 Always in Rage 756287 All buildings and objects are throwable 316022 All monsters are invisible 650867 One time energy 308929 Armies Are 4x Stronger 256806 Turn Military On/Off 135984 Player Indicators always On 677251 Super Energy (Constant Full energy bar) P1 435976 Super Energy P2 603696 Super Energy P3 291680 Super Energy P4 562142 No health power-ups 134615 No Mothra power-ups 413403 No energy power-ups 119702 No rage power-ups 841720 No freeze tanks 913963 Hedorah on every Level
The other way to unlock monsters is to beat the game with certain characters.
Winning With Unlocks Godzilla '90s Godzilla 2000 Godzilla 2000 Destroyah Anguirus Gigan Gigan Rodan Megalon King Ghidorah King Ghidorah Mecha King Ghidorah
Unlock Mecha Godzilla
Beat the game with Destroyah, Rodan, and Mecha Ghidora.
Beat the game with all characters.
Thanks to Revolution readers Max Osman, Gokunks, Jarrett and Mecha-bob!