Ghouls n Ghosts for the nintendo is listed as being amongst the top 10 or so hardest games of ALL TIME -period-.
I should know. I've beaten the game.
I think though that one of the main reasons its difficult is that there is generally a huge number of enemies on the screen at any given time, and that in most of the games you must go through it twice, though why you do that I won't say, since it involves the games plot.
Also, (like the original castlevania) you have to correctly aim your jumps, once your in jump motion, you can't direct yourself.