Ah, Castlevania... The game that, together with Metroid on the NES, made me a gamer (and a game music fan!) 20+ years ago. I never did care much for Super Mario or Zelda back then: CV and Metroid were my first favourite games. I've played and beaten every single CV game since, but the original NES trilogy will always hold a special place in my heart. And while Castlevania 3 is much better and bigger than Castlevania 1 (IMO, tied with Dracula X for the top spot in the series), the first game will always retain it's legendary status.
Regarding CV1's legendary difficulty... I don't consider it that difficult now. Certainly, there are much more difficult entries in the series, like CV3, the SNES version of Dracula X, and - not to forget! - the crazy-difficult original version of the X68000 Akumajou Dracula. There are two tricks that will greatly reduce this game's difficulty level: (1) Enemy placement memorization for the stages (not as hard as it sounds, there are only 6 stages, and they're fairly short), and (2), the Holy Water subweapon with the triple bonus. Especially the latter is crucial: it reduces almost every boss in this game to a complete joke (including the much-dreaded Grim Reaper). The Holy Water paralyzes the bosses, if you for example use it on the Grim Reaper as soon as the battle begins and just keep on throwing it at him, he'll be finished in no time at all. Found a YouTube-clip of the tactic, looky here:
http://www.youtube.com/watch?v=UfzPqDycsVU
The same tactic works wonders against Frankenstein and his &¤)46$€{@{€@!!!!11 little helper, as well as against Dracula's second form.
When I first played Castlevania I found it maddeningly difficult, for a year I couldn't make it past Frankenstein in stage 4. However, I've since played it so many times and perfected my tactic that I can now practically beat it in my sleep. Sometimes, when I'm stressed or bored I'll play through it on an emulator: it's become a very meditative and calming thing for me, almost like it's not even a game anymore, more like some motions I go through to connect with a cherished childhood experience (although I can assure that back then, I certainly didn't cherish getting my ass handed to me by Frankenstein or the Grim Reaper).