The treasury is located deep within the bowels of the Castle. It holds the majority of the Earl's wealth, in the form of gold, jewels, an extremely valuable statue and also the Delberg Castle Map. Access is strictly limited, with the only known key belonging to the Earl himself.
The key alone isn't enough to gain access to the treasury, as the entrance is hidden underwater. The water must be drained from the lowest levels of the Castle using a device found beneath the fountain house. When enough heavy items have been placed inside, the water will recede, revealing the secret entrance beneath the wine cellar.