This was an awesome in debth and personal history of a great series.
Castlevania 2 for NES was difficult for a host of reasons. The biggest one being that there are so many things in the game that you'd have to know either from word of mouth of by buying a Nintendo guide.
It came out long before the internet, so there's parts that you would never be able to beat without someone telling you.
The biggest one that comes to mind is the part where you have to kneel in a specific corner with a specific item equipped that summons a tornado that picks you up.