Fort Riverstride

Map of Fort Riverstride The Quest King Eldrich gives you involves finding the plans for this rambling structure. 

There are 2 Entrance/Exit points from the fort, and they are more or less one atop the other in this map orientation.  
One comes in from the top of the fort while the other comes in from below the fort along the riverbank.  
There are several Secret areas hiding treasure and the Secret area in the SE, off the storage room, will lead you to the lower level where you will find what you seek (if you enter from the top of the fort).
Portrait of Queen Katherine
You need to make your way to the room with a Portrait of Catherine upon the wall.  If you jimmy the Portrait it will slide away revealing a vault that contains the plans Eldrich desires.
The Lever is not essential to achieving your goal, but it can be fun to play with. I wonder just where those explosions are taking place though.
Erathia