Unlockable: Music Player
You must beat the game once. After that, it shows-up under "Extra" on the title screen.
Unlockable: The 50 Floor Dungeon
Defeat the game once and then go searching on the moon's surface for a boulder that's shaped like a head, with a face.
Hint: Experience
There’s a really great way to get easy experience in the Cave of Summons. There’s a monster within called a Summoner, and it will summon certain monsters. Let the creature summon its monsters and kill only the monsters, but not the Summoner itself. If you do this for long enough, you can get an amazing amount of experience by killing hundreds of summoned creatures before killing the Summoner itself.
