After following the clues past the rhino skeleton, heading out through the yard, and past the fountain, you will make your way up some stairs and be standing in front of a picture frame that is the final clue to the treasure map for the quest Cache in the Castle. Opening this picture frame can be a little tricky though. Here is what to do.
In order to unlock the picture frame, you will need to use the buckle above the frame. This will be highlighted blue when you use the revelio spell. You will see a wooden box nearby that you can move around but this is nothing but a distraction from the actual solution for opening the picture frame for the cache in the castle quest.
Once you use the Accio spell on the handle above the frame, it will unlock, revealing a chest inside and allowing you to complete the quest.
