Before obtaining the knife, remove the barrier where the three keys are located.
If the barrier has already been lifted, it goes without saying.
Just slide down to where the keys are.
After walking through the plot, find someone to get a marker map of the position of the three cannons.
The first cannon also has a gorgeous treasure chest. The Thunder Spirit that unlocked the chest was also marked.
After that it is the place with three keys.
Finally go to open the cage and get the drawings.
Hope this post helps you. Thank you.