The new installation Lord Jarol commissioned has been completed. Access to the lower levels of the estate has been concealed and secured by a lock of my own design. The method of passage detailed below should be shared only with those you trust.
Three false wine casks have been constructed to look identical to the rest. One conceals the passage, the other two open it.
To unlock the mechanism, first identify the cask with lanterns hanging closest to them. Each cask requires two knocks upon its face. Two casks, two knocks each. Surely even the most simple minded of your guards can remember that.
If these steps are followed correctly, you will hear the click of the mechanism in the aforementioned third false cask, the one in the western wall that conceals the old passage. It will remain unlocked for a period of time before automatically resetting. If you do not hear the click, I can assure you the fault is not with my mechanism and entirely with your method.
Belrande, Artificer and Locksmith Extraordinaire
P.S. Burn this letter after you have memorized my instructions. Don't post it up in the pantry like my last client. That defeats the whole purpose.