YMMV / Castle in the Darkness

• Anti-Climax Boss: The True Final Boss is quite a letdown when compared to Dark Sorcerer. He possesses only four attacks that can be easily telegraphed and dodged once you memorize what each attack does, and given that he's a Stationary Boss, it's not difficult to strike down his weak point after he performs his attack.
• Game-Breaker: Frog Costume in the New Game Plus initially starts off with only 1 ATK and only 1 DEF, but it also allows the player to double-jump without the need of Windy Boots, has devastating bubble burst magic attack, and every 1500 coins increase its ATK by 1. Thus, with thorough grinding, the player can be able to One-Hit KO nearly every enemy, even the most difficult ones, and turn even the Final Boss or even both Superbosses into, if not pushovers, then still fairly manageable challenges.
• That One Achievement: Completing the Book of Monsters, which pits the player against the wave of 60 different enemies, with only Zombie and Mushbomb being preset in the beginning and the end, respectively, with all enemies between the two being completely randomized. Due to that, the entire challenge becomes a Luck-Based Mission, as the game can just suddenly force you to fight a dangerous late-game enemy right at the beginning. Though the mini-game does provide you with a random weapon/magic/healing item drop to make the challenge easier, you still have very small amount of HP and if you die, the entire mini-game will be reset, forcing you to endure the luck-based challenge all over again until the Random Number God will finally smile upon you.
• That One Boss: The Defender in the Windy Ruins will definitely get on your nerves, as it can be damaged only by deflecting its crystal orb projectiles back at itself, with no other attack being able to damage it, thus forcing the player to wait until it finally fires an orb projectile. And to make matters worse, the player is constantly pushed towards the boss by the wind, and the boss itself constantly fires multiple smaller bullets at the player, thus making the already tedious boss fight much more of a pain to deal with.
• That One Sidequest: Unlocking the Book of Monsters, which requires you to play through entirety of the New Game Plus without taking any damage until you reach the Crystal Tower. Though it's made slightly easier, thanks to your starting weapon being replaced with Cladenets, which inflicts tons of damage per second, and the Frog Costume making the game easier (see the Game-Breaker above), and the fact that you can just die so that when your save is reloaded after revival you'll still have the "no-damage" condition on, you're still required to avoid any and all damage possible until you reach Windy Ruins, obtain Vortex Prism and beat two bosses in Crystal Tower, who by themselves are pretty troublesome to deal with.