Remember when you first in the castle you don't full access to all areas, now that everything is settled, you can go back inside and all the treasures. Here is the item checklist:

• Ether (x3)
• Unicorn Horn (x2)
• Phoenix Down (x2)
• Hermes Sandals (x2)
• Bacchus's Wine (x2)
• Hi-Potion (x2)
• Tent (x2)
• Elixir

In #3 room, there is switch that open a door to a hidden area with 6 chests.

After you collected all treasures, go to #9 and talk to the Baron king then backtrack and exit the castle.