All rooms have a theme, and the last room has a theme, and the theme is being the last room.
Top right: If you got this far, I think you will get this one without clues. Hang in there.
Bottom left: The castle is old. The animals in the front are yaks. The puzzle describes what the next puzzle is.
Bottom right: Who's holding a what? The puzzle describes what happens when you finish it.
Yellow room top right: All four words in this puzzle rhyme. The second solution concerns the central character, but also contains the name of the beverage being consumed
Blue room top left: One of these solutions is literally just two of the depicted objects
Good luck!