From the Edrondepot, go down the drain in the south-west corner of the castle (here).
There is only one way to go, so follow this cave down and up until you emerge in a grassy area (here).
Go to the Troll Cave to the north-west (here), and go down the hole. Be prepared to face up to 20 Troll Champions if no one has been down there for a while. For low level players it's advised to move in slowly, there's a small passage where only one of the Troll Champions can attack you.
Follow this path to the north and east, and go down the hole: