These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
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Anticlimax Boss: The boss of Hell Temple, The Boss. Not that he's particularly easy, but Hell Temple is so nightmarishly hard that this guy's a cakewalk in comparison.
Demonic Spiders: Averted in later stages, but played straight early in the game, where if you don't have the Waterproof/Heatproof Cases, anything that can knock you into water or lava is a Demonic Spider.
Erratically moving Podobo ripoffs in the Inferno Cavern, I'm looking at you.
The Water Leaper (AKA Surprise Fish) from the Spring in the Sky is probably another candidate, considering that its spawn points are tough to see and it only jumps out at you when you're nearby. If it hits you, there's a good chance you'll end up in the water.
Tougen—large, red, turtle-like creatures that fire hadokens at you when they face you. They're impossible to avoid damage from, let alone kill, until you get the Silver Shield. And they always seem to be found in tight spots where they can't be jumped over.
Ear Worm: The Hell Temple music. It's fully capable of causing Vietnam-like flashbacks.
Fan Nickname: "Catball" for Cait Sidhe, a cat-like enemy that rides on its ball, and the notorious "Surprise Fish" for Water Leapers, which are fish that jump out of waterfalls unexpectedly.
"Jackrabbit" for the Nuckelavee mid-boss in the Spring in the Sky.
Faux Symbolism: Lots. There's a large amount of ancient religion-style symbolism and implications in several levels and puzzles, and the final boss is all over this.
The official blog expressed concern over this for the WiiWare port, but fan response has been overwhelmingly supportive of keeping all of the symbolism intact, misguided as it might be.
Freud Was Right: Spring of the Sky, map A-4◊. Look at the background murals, and notice that there are two legs, and something between them. Though this is desquickified by background information, as Uonin (this species of fish-men) have the habit of carrying an iron air tank in their crotch. It's still as suggestive, though...
One of the puzzles is to impregnate a female statue with energy-like sperm in the "male" Sun temple, then to sleep in front of a mural of a uterus in the "female" Moonlight Temple! Freud might have been on the dev team.
Goddamned Bats: Pretty much everything. Of course, the bats themselves are particularly annoying, especially when they blend in with the background. Which they do much more often than you'd think, as a surprising number of areas in the game have blue backgrounds, including Hell Temple. This may have been deliberate. But almost everything eventually becomes a goddamned bat.
The Water Is Blue; the bats are blue; the bats can fly underwater. ARGH!
Goddamned Boss: Several minibosses have the tendency to knock the player off the screen, causing to restart the battle. Others are just hard to hit.
Tiamat, who among other things spawns Goddamned Bats - which don't even give any experience for healing. She's only saved from That One Boss status (even by La-Mulana standards) by there being an easy (by the aforementioned standards) strategy to beat her.
Bahamut also qualifies, as half the time when he jumps out of the water he will use an attack that is 100% unavoidable if you don't jump into the water (which hurts you whether you have the Scalesphere or not). Even if you come prepared with a lot of life jewels and the body armor, he can still be a pain in the ass to fight.
The problem with Bahamut isn't so much the monster itself as that boat is damn hard to control. Once you get the hang of the boat, Bahamut is fairly simple.
Several of the minibosses in the Dimensional Corridor could count. One in particular is Dragon, who is an integral element in That One Puzzle, and renders the puzzle nigh-unsolvable if he's killed.
Good Bad Bugs: In the Endless Corridor it's possible to jump through the ceiling of the main floor and appear in the bottom floor. This allows players to bypass the floor with all the block puzzles.
It was possible in the remake to glitch through a wall in the Tower of the Goddess to enter the scale room without unlocking it. This has been fixed since version 1.5.
Deceased Crab: Maybe I'll finally find a use for those 96 weights I have. I guess so! Maybe they'll have a room where I have to use 100 weights somewhere. Here, see all those things? Put weights on all of these pedestals. Just what I've come to expect.
Xelpud jokes in the remake that even if the game is popular there will never be a sequel. Thankfully, he's wrong.
Take That, Scrappy!: Several references to "bat curry" are made in the remake. Furthermore in the remake, the Scripture item will render you immune to bats.
That One Boss: Several. The final boss counts, given that you have to fight a five-form boss, with a final dungeon beforehand, with said dungeon also including a boss of its own. In addition, almost every boss has a weakness to a particular weapon or subweapon; if you haven't got said item yet, it's almost guaranteed to be a tough fight.
Tiamat is a nightmarish fight. As you're attacking the symbols around the room, she'll spawn Goddamned Bats, as well as whipping you with her tail (which is nigh-unavoidable and knocks off a lot of health) or attack with Prehensile Hair (which is also difficult to dodge, but at least is telegraphed well in advance). Without the Castlevania/Mahjong Wizard ROM combo or all the Life Jewels, it's almost unbeatable.
Bahamut also proves to be a huge Jerk Ass of a boss. If he decides you're going into the water, you're going into the water, and it's made all the worse when you consider the water hurts you whether you have the Scalesphere or not and it can be very difficult to get back onto your little boat at times. Half the fight is just trying not to get wet.
Viy presents a challenge of great difficulty. The constantly-scrolling screen, eye beams, and four tentacles shooting bullets at you makes his fight a nightmare. It's tweaked in the remake, with the tentacles being longer (thus easier to hit) and not shooting bullets anymore, but Viy launches his attacks with less warning in advance, and hitting his eye no longer flinches him out of his current attack. And coming into direct contact with his body saps your HP fast, ignoring any Mercy Invincibility you may get from his other attacks.
Palenque spams attacks everywhere and is difficult to go through the spaces in the pillars in time. If you hit him, you lose almost half your health. In addition, he's got a tiny hitbox, and being an Unexpected Shmup Level, that makes things...difficult.
Palenque becomes a nightmare for players trying for the no sub-weapons achievements. Positioning Lemeza so he can hit Palenque while not taking damage from his ship or his body (the latter drains Lemeza's health constantly) is tricky enough, but of course Palenque keeps shifting vertically, forcing the player to try to line up again.
Sakit is the second boss in the game, and many new players find him to be a brick wall of a boss. For starters, he's immune to subweapons (except the extremely expensive pistol) and only capable of being hit at certain times (namely, after his Rocket Punch). His other attacks can be quite hard to dodge and are lethal if you don't have a few Life Jewels. If you found the knife before taking him on, he's a reasonable challenge, but the starter whip is so slow and weak you'll be chipping away at his health for a while if you have only it.
The remake makes him a bit easier. Not only can Lemeza now destroy the rocks the fall from the ceiling, midway through the fight Sakit becomes vulnerable to sub-weapons. Plus a fully-loaded pistol is just 100 gold for players who still have trouble.
Baphomet, in the original game, had relatively feeble attacks for a late-game boss and could be taken out easily with a few bombs, or by standing on the platform above her and just chucking spears. The first thing she does in the remake? Smash the platform above her head, forcing you to fight her from the sides. And not only do her attacks cover a lot more of the screen (the lighning bolts in particular) this time, halfway through the fight she stops sitting around and makes a One-Winged Angel transformation.
That One Level: Tower of the Goddess. Make one slip when climbing the tower, and get sent back to the bottom. Just adding a safety guard at the bottom left/right corners would help a lot.
The Gate of Confusion/Illusion, since, as its name implies, its whole theme is having rather obtuse puzzles.
Chamber of Birth gains some ire as well, as some feel it has puzzles comparable to Confusion Gate, only without being as honest about it.
The puzzle on the second level of the Infinite Corridor. Not only does the lantern arrangement require you to know the La-Mulana numbering system (which is only vaguely hinted at in game; there's no indication anywhere those symbols are anything other than eye candy), but even if you know the answer (2012, it's in the manual), the game's not guaranteed to acknowledge it. You can solve the puzzle all you want, but the third level will only open up when the game feels like it.
It's not much easier in the remake: You now have to pass through 4 lanterns in ascending order. The first two can be done by looking at the seals you have, but the last two are hinted by Mulbruk which sometimes doesn't show. Worse still, people assume you need all of the lanterns, which is not the case. On the plus side, at least it acknowledges the solution without fail, and any incorrect lanterns in-between the actual answer are ignored.
The confusion gate puzzle where you have to destroy all five pots at once. Possible, but if you miss, you need to wait 3 minutes for the lamp to recharge. This is a little easier in the remake since you can recharge the lamp in the nearby Tower of Ruin.
Not only does the remake let you recharge the Lamp more quickly, but you can also use the Bracelet to destroy the pots in 1 hit (in the original each pot took 2 hits whether Lemeza had the Bracelet or not).
Almost every late-game puzzle can be this if you don't have certain items. Doing the block puzzles in the Endless Corridor without the glove is nightmarish — it doesn't make the puzzle any harder, just slower. Much slower.
Waiting for that ***ing lizard to walk in front of the light. This is also easier in the remake as the lizards seem predisposed to chasing Lemeza, making it easier to bait towards the light.
That jumping puzzle for the Life Jewel in the Dimensional Corridor. While the jump puzzle itself is a little easier in the remake, actually getting to that room to do it is now much harder.
Woolseyism: There is no MSX game with the name Castlevania. The game was released in Europe (though not North America) under the name Vampire Killer, but translator Ian Kelley calls it Castlevania in-game because it's a more recognizable name. Uranai Sensation got renamed to Diviner Sensation to make more sense to non-Japanese speakers.
Some of the items in the remake's official English translation have different names. For instance, the Flywheel has become the "Cog of the Soul", presumably because a flywheel being used to create life seems a bit strange. The Wedge is also now called the "Djed Pillar", which is appropriate because it basically is one in appearance.