There are things to discover in both wings of Troia Castle after you are done speaking to the Clerics in the center. You'll find Edward on the west side, and he'll give you the Twin Harp—do not forget this, or you won't be able to defeat the Dark Elf! On the east side, you'll find three staircases leading to Treasure. You won't be able to get the bulk of it until you retrieve the Earth Crystal, but don't miss the Ruby Rings at the very least. As you learn here, you'll need non-metallic Weapons in the Magnetic Cavern, so they will come in handy quite soon.
After you return with the Earth Crystal, note that you will not be able to use your airship to go anywhere until after completing the Tower of Zot. (You can use a black chocobo if you need to fly somewhere, though.) You can grab the Items in the left-side basement at this point as well.
After you return with the Earth Crystal, note that you will not be able to use your airship to go anywhere until after completing the Tower of Zot. (You can use a black chocobo if you need to fly somewhere, though.) You can grab the Items in the left-side basement at this point as well.