This was probably the hardest mission I have done so far. The prima guide didn't help at all, and it seemed like everyone was stuck on this as well when I looked on the forums. I wandered through this house for probably close to two hours before I realized what I had to do, and it was so simple I facepalmed. I have decided I will only show the direct route you will use to complete this mission, and not show you every room of the house I wandered through. This is because a majority of the rooms have nothing in them, I only got into those rooms with zenporting (which you do not need in this mission after all), and they have nothing to do with this mission.
Mission: Locate the keystone in Chateau du Landgraab in France needed to explore the rest of the Dragon Temple in China.
Examine the hole in the ground.
It disarms the traps.
Go through the door on the left.
Watch out for these traps here. Go through the double doors.
Go up the stairs to the 3rd floor.
Put the keystone in the slot.
Don't take the statue off the display!! The electric traps will be activated. You can take the treasure off the left display though.
If you did take the statue, put it back by clicking the display and then finding the statue in your inventory. This will deactivate the traps.
*I have been informed via comment, that you can place anything from your inventory onto the display to deactivate the traps*
Collect the treasure off the bedside table.
Continue through the bathroom...
Walking into this room will activate the hidden door to appear.
Go down the stairs to the basement.
Watch out for the fire trap on the wall to the right.
Go through the door to the left.
You can explore the other rooms if you want to.
You can harvest some of the grape vines.
Grab the keystone from the glowing chest, and the keystone off the table.
Go through the door at the end of the hall, and the hidden door will activate.
Another hidden door here.
Collect the treasure. You can now travel back to China to complete the mission.