Pretty much every Demon. The Earth Demon gets an honorable mention for being in the fourth screen of the game, and with attacks far more damaging than anything you have yet encountered, or even anything you will encounter up until the first boss. Tough to dodge, too. But the one that is by far the worst is the Ice Demon. It spams an attack that gives next to no warning, requires I Wanna Be the Guy level platforming skills to dodge even if you know where it's coming from, and if it hits, you're as good as dead. It freezes you in place, which means you WILL get hit again, and when you do, it's for three times the damage. Think you can just whip the attack away like you could with the Fire Demon? Yeah, that'll cause it to shatter, all right. Into shards that are equally as deadly as the original attack, only even harder to dodge. The non-elemental Demon, a seemingly intentional Boss in Mook Clothing to the point where the room at the end of the Bonus Dungeon consists of just one of them, isn't half as deadly as this one attack.
The game is full of Demonic Spiders—a lot of the Armors have very annoying attacks and appear more frequently than the demons. Flame Armors have a simple sword swing, but the hit range is way larger than what you'd expect, Stone Armors throw three boulders that get progressively closer and do splash damage, Thunder Armors fire homing orbs, Ice Armors throw two spears, one high and one low, both of which freeze you (see the Ice Demon above for all the fun being frozen gets you), and Poison Armors shoot tall clouds of gas that are difficult to jump over. All of these attacks hit really hard and make it tough to get close enough to whip them. None of their attacks can be countered with the whip, they often take four or five hits to kill, and if you revisit an earlier area late in the game, the Armors that were there are replaced by more powerful varieties.
The worst armours are probably the Dark ones, mostly because they fire these orbs at you. Easy enough to avoid right, except for the fact they're always places where you're trying to get up, and you'll get hit over and over and over... That devil guy who appears lategame is a royal pain as well and nigh impossible to kill without sustaining heavy damage as he just spams loads of random crap at you. It's tough to even get by him!
Foe Yay: Hugh's antagonism towards Nathan, can be interpreted as sexual tension. Nathan on the other hand doesn't react aggressively at Hugh's actions and taunts, but is rather worried about his change in behavior, making it Ho Yay on his side.
Fridge Logic: You can slide continuously, but you need a specific magic item to be able to run.
Crosses. They deal about as much damage as your whip, but can actually hit at least three times if you measure the distance properly, since they stay in the air for a while once they reach their maximum range. They're very hard to find, though, and it's easy to get one replaced with another subweapon by accident.
The Jupiter+Manticore combination forms a poisonous mist around you, which instantly destroys most of the projectiles caught on it, making bosses like Adramelech, Camilla, Death and Dracula's second form easier. It is also handy in discovering hidden rooms.
The Uranus DSS card, too, which can summon creatures to deal massive damage to everything on the screen, and one of them is the only healing spell in the entire game. (Other than Jupiter+Mandragora, although that only works via slow regeneration) You'll only find it late in the game, though, and even then, only if you have a guide.
Thief Mode, which turns the frustratingly low Random Drops back into your favor pretty fast. Although it bumps down your other stats, making the beginning of the game harder.
Magician Mode, where your intelligence stat is so high that you fully restore mp in a matter of seconds. By level 33, your mp recovery rate is so high that even your healing magic doesn't drain your mp at all. There is also the Pluto+Unicorn combo, which grants you full invulnerability at a cost of not being able to damage enemies, which is hardly an issue. In other modes, you only get the Unicorn card after the Battle Area, so it doesn't really get used much, however in Magician mode, you have access to it from the very start of the game.
Fighter Mode puts your attack strength and defenses through the roof at the cost of not being able to use magic. But who needs magic when your enemies are about as effective at hitting you as a blind man is at driving and you can kill any of them in 3 lashes of the whip maximum?
Through a glitch, you can use any DSS Card combination after you get your first one. After casting the spell but before the effect kicks in, change the DSS combo to whatever and you'll be able to use that combo. You can also change your card combination to something that boosts your strength while in the middle of a summon spell to greatly boost its' damage.
The Bonus Dungeon is supposed to prevent you from casting magic. They implemented this with an insane Mana Drain effect, but you can still chug a potion or five and quickly cast something before you run out of MP.
Though you are prevented from attacking enemies while cursed, you may still damage them by sliding into them. This doesn't completely alleviate the problems with the areas with curse-inducing enemies, but it does help some.
Ho Yay: As said above, Nathan really cares about Hugh and becomes livid when Camilla reveals how she and Dracula corrupted Hugh. Dracula's rant about bringing out Hugh dark side makes Nathan very angry, arguably even more than kidnapping his and Hugh's mentor.
Narm: Dracula is twice as big as his own coffin when Camilla wakes him up at the beginning.
Nightmare Fuel: Hugh, especially during his fight against you. You can't imagine when you are being mind-controlled by Dracula and not knowing what the hell you are doing until it is too late.
There is no "suspend" feature. Coupled with the lack of save point density, playing this game on short breaks may not be such a good idea. Konami would learn their lesson come next game, however.
The cards being random drops from specific enemies that are never hinted to in game. It is entirely possible to complete the game with a large majority of your card collection incomplete if you don't use a guide.
Scrappy Weapon: Averted with usual offender, the Dagger. It will quickly become your weapon of choice when you are in Archer mode (but ONLY in archer mode). That is because all of your subweapons get a damage boost, making it a strong weapon. The second is that the upgrade at no cost, is the homing dagger. And it is VERY useful. It ties with the cross, as the cross does more damage and hits more targets, but the dagger homes. Played straight with every other mode though where they are virtually useless.
That One Attack: Dracula's One-Winged Angel form will start spamming a charge attack at low HP which takes up half the screen, will wipe out almost all your health and is virtually impossible to antipate the first time (though his glowing might be an invitation to get the hell away, you need to go up specifically). At low enough health he will use nothing else. And don't get started on his Goddamned Bats which makes it nigh impossible for you to get close to and whip him...
The minotaur room in the arena. Seven of them, each with 700 hp and a ridiculously powerful attack that reaches halfway through the screen, both horizontally and vertically . Halfway into the dungeon, far away from the save point, no magic, and nowhere to run.
Zombie. Dragons. You don't have a lot of room, they like to swing wildly and drop random debris from the ceiling, and when you kill one, the other literally devours the other to gain 1000 hp. They're also above you, making attacking them awkward. The Ax is your friend (for once).
What happens you combine a level that's heavy on the jumping with Medusa Heads? This trope.
And the goddamn Chapel with the marionettes EVERYWHERE which curse you, making you unable to attack, and worse still, bloody swords which move in an erratic pattern and are hard to hit even when you can attack. Both enemies are spammed like hell.
There's also the Gallery; a segment that's filled with enemies that can poison you with little room to maneuver past them all.
While the Battle Arena is hard through out, two rooms stand out as difficult even by the Arena's standards - one is wall to wall Minotaurs, and the other is filled with Poison Armors and Bloody Swords.
They Wasted a Perfectly Good Character: After earlier Castlevania allowed the player to play an alternate mode as Richter and started a trend in the series of letting you play as characters other than the main character in most of the games afterward, it becomes rather disappointing that Circle of the Moon doesn't let you play as Hugh, especially since the character's entire hangup involves trying to be better than Graves.