At the Main Menu press B A B A B A Left C.
Wait for the 'Hurry Up' to appear on levels 6 11 and 17 and a letter will appear. If you spell ROD you'll gain a giant road which makes you invulnerable and turns anything you touch into a diamond. It will also return the dragons to their human form.
Play as Humans
At the Main Menu press Left Down Up B Right C A.
At the Main Menu press Left Right Left Right A B C B.
At the Main Menu press Up C Right B A Left C Down.