To unlock the secret Dragon class, you must purchase the last upgrade to the castle.
Once "Invuln Time Up" is unlocked, a flag pole will appear at the top of the castle.
Press [Up] to select the "Beastiality" upgrade for 5,810 gold. Purchase this upgrade
to unlock the Dragon class. The Dragon class can always fly. While flying, the Dragon
can shoot mid-range fireballs that consume mana when activated. The Dragon cannot 
attack with the sword while flying. If you have only upgraded strength, the Dragon
class will not be as powerful. It also requires magic-based equipment to improve stats.