Not a key, a spell if I remember correctly.
You can read it but it wont do anything.. after that you interact with the barrier at the castle once more it will have another option to select that allows you in.
I can't find any spell in the forge...
Edit: Found it after reloading a savegame before the forge. The corresponding "spell" is called "orders" and it's heavily bugged! If you have another paper called orders in your inventory the new one just goes missing...
