Once the Castle has opened up to you, make your way to the upper-right area with three ropes you must jump between to climb to a level door. To the right of the ropes, there's an invisible hole in the wall you can jump through (you can see a piece of wood jutting out there).
This screenshot shows the location:
https://www.trueachievements.com/gamescreenshot.aspx?gsid=15...