So what is the rule of thumb for the NES? The rule is this, and it goes for many games out there, if the level is easy, the boss is hard, if the level is hard, the boss is easy, the exception always being the first level or two. The story for Castlevania is simple. SImon Belmont is out to slay Dracula, just as his Grandfather Trevor did and Great Grandmother Sophia did. He must go through the Horrors of Draculas castle facing along the way the Gorgon, Vampire Bat, Mummy, and Frankenstein creature. Each level gets progressivly more difficult, but follows the ruel of thumb. For example level 4 is not all that hard, however the boss (Frankensteins monster) is very difficult to beat (mostly because of that damn imp). However level 3 is quite challenging but the boss (Twin Mummies) is not that hard to beat.
Overall the game is challenging, but not impossible. It will take time, but it can be beat.