Doshin the Giant Hints & Cheats

Go to the game selection screen and in the options menu enter these codes:

To unlcok the secret Mystery Monument you have to build the LOVE or HATE versions of the first 15 monuments.

All the monuments do not have to be built at once but it must be on the same savegame. Once they are unlocked in the Monument Museum destroy one of the existing monuments and the Mystery monument will be built.

This will trigger a cut-scene and unlock the bonus maps and alien level.