Complete the mission by searching for their "Lost Ancient Books", to collect two Premium Chests, a Super Chest and the "Lost Songs (I and II)" music pages.
Activate the challenge south of the North Teleport Point in Fading Castle: Middle Floor, then collect 15 Water Elementals within 60 seconds to receive a Super Chest.
Go up the stairs next to the previous challenge to reach the Fading Castle: Upper Level. When you arrive, you will have to pass the Mystery Bookshelf challenge to enter. Once solved, defeat the enemies hidden inside and collect a Weird Chest.
Glide down from the Fading Castle: Upper Floor to the Middle Floor to collect another Excellent Chest in the location shown below.
Hidden behind a Mystery Bookshelf Challenge is a Super Chest. Solve the challenge by finding its "Missing Ancient Book", floating near the stairs, and placing it in the bookshelf to get the treasure chest.
Here, you must complete three performances in the "Surprise Stage" section at the Faded Castle: Lower Floor to receive valuable items. What you can get includes a Super Chest, Rare Chest, and a Tuba Mirum music score.
The Final Treasure Chest in the Fading Castle is hidden beneath the water and can only be unlocked by playing the correct notes for each Harmonic Pipe.
Just in such an area you can earn quite a few chests and open up Adventure Exp, primeval stones, some amount of Mora, random weapons, holy relics and more.