There are several bonus stages in Dungeon Keeper, but one of them can be found without doing anything. To access it, set your PC's date to any day with a full moon then start the game. The bonus stage will appear as a flag with a circle in it.

Go into the data file, then open up the creature file and you can change information about creatures and spells around. Ex: You can change the imp spell to where imps cost nothing.

At the main menu screent type: please give all