In Game Description
Key to the giant door leading out from the uppermost floor of Archive Tower, which now serves as a prison.
The key's design resembles not the key of a prison; indeed, the tower was once no prison at all, but a trove of precious tomes.
Availability
Found in the prison section of The Duke's Archives. The key is in a chest behind the gramophone, guarded by two Serpent Soldiers and a Serpent Mage.
Opens
Unlocks the large doors at the ledge above the ladder, at the top of the stairs in the prison section of The Duke's Archives