Breather Level: On the Alucard path, opting to finish Stage 5 rather than going to the Sunken Ruins takes you to the Catacombs beneath the castle. It's unusually short, has fairly simple challenges, and both Grant and Alucard have ways to make it even shorter. Even the boss (Frankie, who also appears in Stage 5 of Sypha's route) is pretty easy.
The Castle Entrance is this on all paths, despite the fact that from this point on you die in four hits. It's short, light on the bottomless pits and tricky jumps, and allows opportunities to get lots of hearts. The boss is certainly not easy though, considering it's Death.
Demonic Spiders: On the hard mode of the game, the Skull Heads that replace the Medusa Heads. They're far worse than the Medusa Heads and Bats, flying in an incredibly erratic and unpredictable pattern, while they're capable of flying up or down the complete screen worth. They'll additionally always deal 4 bars of damage when they hit, and some level screens that didn't have the Medusa Heads have the Skull Heads flying around on the hard mode.
Ensemble Darkhorse: Alucard was a relatively minor character in this game, but he evidently had enough of a fanbase to later become the main character of Symphony of the Night.
Game Breaker: Once you unlock Sypha, there's little reason to switch back. She may not have any special jumping or controls like Grant or Alucard, but her spells are very powerful. Fire's easily the least helpful as it just shoots in a straight line, but it's still a strong attack. Ice will freeze enemies into blocks that can be killed with a single attack there after and can be made into a platform if you're quick, it also freezes water hazards which can be helpful against the currents. Lightning fires in all directions and will homing in on any enemy, including Death and Dracula, which will track them even as they're teleporting or dashing across the screen. Her staff may have shorter range than Trever's whip, but its animation is quicker and can be rapidly spammed.
Good Bad Bug: When fighting the Doppleganger, when you switch characters, the Doppleganger will transform alongside you. However it doesn't transform during the switching process, instead taking a second to transform after you switch, where it is completely vulnerable. A player can exploit this by switching, taking a shot at it, then switching before it can attack or move, and repeating the process until it dies without ever getting a chance to actually fight back.
Scrappy Mechanic: The stairs. Stairs severely limit your mobility, and due to pressing up at the base of stairs or down at the top of them causing you to go on them, you will inadvertently find yourself going on the stairs when trying to crouch or use a subweapon near them, often causing you to needlessly take a hit. The control conflict with subweapons also means you can't properly use subweapons on stairs, as many of your subweapon inputs will not go through, which when combined with the aforementioned mobility issues, often leaves you a sitting duck on stairs. While these problems were in the original Castlevania, stairs are a lot more prevalent in this game, and there are many vertical platforming sections that requires you to go on stairs unlike in the original game. Then there's the fact that one of the playable characters, Alucard, inexplicably can't use his basic attack when on the stairs, leaving him completely defenseless on stairs.
The Doppleganger, who'll give you quite some trouble unless you exploit the aforementioned glitch to defeat it. If you're playing a Trevor solo playthrough and thus won't have access to the aforementioned exploit, you better hope you get to it with Holy Water intact.
Death is this as usual. He flies around summoning scythes and you die in four hits like the encounter in the first game, only he has a second form this time round. Fortunately, the second form is easier than the first form, and the first, while really hard, is not as brutal as in the first game due to you having more options and room to manouver, while he also has a bit less health (requiring 12 whip hits to kill here compared to 16 in the original).
Stage 7-5. The only way to advance was to wait for the falling blocks to form a platform for you to ascend. If you got hit by a falling block, you were likely dead, and had to repeat the process from the start. If you brought Alucard along and had enough hearts you could just fly up and skip this part entirely, though if you brought Grant or went solo, you're SOL.
Stage 7 as a whole could count, seeing as it's about as long as 2 normal stages. The best part? The hardest parts are AFTER the falling blocks, so every time you game over, you have to go through them again. It also has a fairly difficult Sequential Boss at the end, where you had to fight a pair of bossified mummies (who are especially bad on the NES version), a Cyclops, and the Leviathan, a Damage-Sponge Boss who had over twice as much HP as most other bosses and marks the point in the NES version where enemies start dealing 4 bars of damage, while being one of the few enemies in the game to deal 4 bars of damage in the Famicon version.
There's also Stage 9, which is almost all the different challenging segments remixed into one big ass and really difficult stage. The fact that you die in four hits at this point on the NES version is the icing on the cake. And it ends with the aforementioned Doppleganger fight, a really difficult boss fight if you don't use the aforementioned exploit to beat it or don't make it to the fight with Holy Water intact.
Viewer Gender Confusion: This was especially common with Sypha before the days of the internet. The only way you found out she's a girl is beating the game with her as your partner. Considering how difficult the US version is, that's no small feat for kids of the day. The fact that the text leaves the prompt as "Take him with you?" on her select screen doesn't help. In Japanese, it was perfectly fine using gender-neutral pronouns.