Final Fantasy III/Hein Castle

Castle Argus treasure room
Check the fourth candle from left for a secret passage to the east. You will then come in a room with 3 chests (3000 Gil) behind a wall which can walked through. Go upstairs and you find 6 more chests (4700 Gil). Go upstairs again and go left until the door. Unlock it with a thief in your party, if you don't have one then change someone to a thief and place him at the top of the party.

Check the dresser to give way to 3 chests (Killer Bow, 20 Light Arrows and 20 Medusa Arrows). Exit the room and go right to another door. Unlock it and you are in a room with 6 chests (Book of Fire, Book of Light, Book of Ice, 20 Fire Arrows, 20 Ice Arrows and Scholar Robe)

Go back to the room with 4 candle and go upstairs.

Now you're done here so exit the castle and go southwest to a forest called...

Living Woods
You will meet some fairies there and a crater where the elder tree was. The fairies tell you that Wizard Hein from Castle Argus came and placed a curse on the elder tree and took him. The elder tree is transformed in a castle and wander the desert near Tokkul aimlessly. They also tell you something about Hein's weakness. Now go back to the town of Tokkul.

Hein castle
Most enemies are undead, i.e. weak against Holy and Fire, as the Scholar can confirm. Let the Ranger use his Holy Arrows freely, as this dungeon is the last one in the game with a large number of undead enemies. The Knight and the Scholar should equip fire weapons: the Salamand sword and two Books of Fire, respectively. Switch to any other arrows and sword when fighting against enemies that are not undead (access Item menu during battle). The Thief should always try to Steal instead of attack.

BOSS: Hein
The Scholar should repeatedly Study the weakness of Hein. Then, open the Item menu and change the weapons of Scholar, Knight and Ranger to match Hein's weakness. The Thief, instead, neither has elemental weapons nor he can steal much from a boss: let him use the offensive items he stole during the various battles.