Before trying to solve this one be sure to have read my info on room time cause there are some nasty Frankenstein's monsters you've got to evade with correct timing as demonstrated in my longplay.

This castle appears to have some weird doors. First there are the two doors on the bottom of the exit room both of which lead to the room below only one to the top and the other one to the bottom.

Then there's the right bottom door in room 5 (counting from top left to bottom right) leading back to the exit room too which (intentionally?) makes for a nice shortcut.