Completing the Quest Mode in One Player unlocks extra modes and cheats as well as levels for Two Player mode.

Unlockable	How to unlock
God Mode option	Complete New Game Plus mode.
One Player - New Game Plus mode	Complete Quest Mode.
Two Player - Gem Hunter mode	Complete Quest Mode.
Two Player - Made-of-Glass mode	Complete Quest Mode.
Unlock levels in Two Player mode	Completing levels in Quest Mode unlocks the same levels for all Two Player modes.