After obtaining this key, jump down first, defeat the enemies below, and head left.
Insert the key you just obtained; there is a woman in the cell.
After talking to her, the quest "Secrets of Baishan" we accepted on the bridge will update.
Now turn back and go up to the right again.
This time, move a bit forward before jumping down.
You can obtain the "Thief Camp Dungeon Key" next to the corpse.
