Slay the Purple Squid, the key holder!
Destroy 3 or more Giant Squids
Collect the coin in the rapids inside the white stone circle.
Find a coin hidden by a dungeon gnome in Labyrinth Town.
Collect the coin in one of the saw rooms.
If a wizard is powerful enough, he can even defeat a Basilisk.
Even the toughest can be cracked.
A coin in the palace? Of course it belongs to the King!
When drunk in a tavern sometimes a gold coin must be stolen.
Someone must have dropped this coin, maybe someone observing the broken bridge.