Final Fantasy 4 Advance - Platform: Gameboy Advance

Completion bonuses:
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Complete the game and continue from your cleared saved game file,
then explore the surface of the moon until you see a face-shaped 
rock. Enter it to access a fifty level dungeon. The Music Player 
option will also be unlocked in the Extra menu. 

1991 Developer Team Office:
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When you get to the Underworld for the first time, defeat Golbez 
in the crystal room. Get access to the armory, then go down to the
weapon/armor room. Go behind the pillar that is between the two 
shop desks. Move up from there to enter the hidden Lali Ho Pub. 
Then, move to the right to the dark spot on the floor. You can move
through the wall from here. Climb down the stairs to reach the hidden 
1991 Developer Team Office entrance. Enter the hidden area to talk to
the game developers. 

The 50 Floor Dungeon:
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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.