"This castle guards the key that guards the tomb that guards the bones. And don’t think the guards will allow the brass ring to be grabbed by just anyone." - In game description.
This is the final dungeon before the boss battle at The Tomb. The dungeon is littered with high level weapons and armor, as well as a fair amount of treasure.
The angel is named Evelyn and she can be found on the north island in a lake in the northern part of the map. You can reach her by going through a passwall in the second to the last room on the left in the main hall to reach a teleporter to the south island in the lake. "Two Ship in a Bottle" spells are provided to get you to the north island.