Invincibility
Beat the game twice and then go to multiplayer, once you have one three games of multiplayer, play single player, you will have invincibility.
Gremlin fun
On one of the levels when you are in "Hell", you will come across two gremlins trapped in cages. There is a gremlin that is in the hanging cage with the lever in front of it, if you press the lever the cage will drop down a little bit, hop down with it, and there will be another lever and a furnace. Press the lever and a giant flame will come out of the furnace and fry the gremlin! Its pretty funny.
More Rune Power
In Sutlr's Sridge, if you jump on the right ledge at the beginning of the level and go all the way back to the right, there is a ledge just below you. If you look straight down you can see it. Go to the left on the lower ledge and jump on the chain in front of you and then climb to the top, you can jump to the next chain. on the second chain, go all the way up, and jump to the platform with the rune of lesser power on it. Now, if you look back to the ledge you came from, you can see another rune of lesser power. to get back to the ledge, repeat the sequence with the chains starting with the second one. expect more secrets, cheats, and helpful tidbits as I progress.
God Mode
During play, pause the game and press Square, Circle, Left, Right, Circle, Square.
Max Rune Power
During play, pause the game and press Up, Down, Left, Right, Square, Circle.