Right from its opening minutes, The Hidden Dungeon Only I Can Enter throws a stack of clichés at you, as if it's trying to compete for the title of most generic fantasy anime ever. It may not technically be an isekai, but it's got all the hallmarks of a LitRPG-flavored isekai setting: It's a world of swords and sorcery, where characters have levels and skills. Our protagonist enters a dungeon, meets a mysterious girl, and gains a skill that allows him to create or edit any skill he likes. To power himself up, he has to do horny things with girls. (Of course.) He even goes to Hero School and joins an adventurer's guild, where he quickly displays how overpowered his skill is.
There is not a single novel aspect of The Hidden Dungeon Only I Can Enter, but at this point the appeal of this particular collection of tropes is the familiarity of it all. Most importantly, it's light-hearted. As this anime goes through the motions of its predictable plot points, there's no hint of edginess or self-importance. While not as obviously parodic as KONOSUBA, the stakes are low and the situations are humorous. It would be a stretch to call the lack of pretense "refreshing," but I watched the whole show with a smile on my face. That has to count for something.
Still, this can't be described as a good anime even by a generous assessment. The production values, for one thing, are atrocious. The characters barely move, and the quality of the compositing is so low that it looks like an anime from the mid-2000s. Any attempts at action or emotional moments are immediately quashed by the poor animation and stilted presentation. The characters have only one or two personality traits, and that's if you count the size of their boobs as a personality trait. Even as a comedy, many of the jokes are so played-out that they're just as likely to inspire a groan as a laugh.
The anime starts on its worst foot; in its attempt to cover all the plot points for its needlessly elaborate setup, the characters end up coming off as stage props. Noir's childhood friend Emma is the most egregious example of this. She's probably the most likable character in the show overall thanks to the stellar efforts of voice actress Miyu Tomita, who injected a lot of genuine sweetness and affability in her performance, but that isn't evident from her first appearance. She's introduced by her breasts before her face (this is the case for almost every major female character in the anime, by the way). Then, in their very first scene, she suggests that she and Noir kiss as a way of soothing his headaches, and they just... do it. The moment is striking for its complete lack of buildup or chemistry, despite the romantic violin track playing in the background.
After the protagonist enrolls in school and the story settles into a comfortable status quo, the charms of the storytelling become a little more pronounced. The highlights are the absurdist dialogue and situations, like when Noir has to do sexy things with Emma to power up in time while a giant monster rabbit skulks around them, or when he and his harem have to appeal to a group of misogynistic guys who have made a regular colosseum sport of yelling at guys with harems. The translation in both the subtitles and the English dub do a great job of capturing the straight-faced ridiculousness of the lines.
It's also notable that, despite having a plot that facilitates sexual content, most of the fanservice is honestly on the tame end of the scale, like shots of jiggling breasts or the girls' chests pressing against the protagonist's body as he is getting hugged. There's a running theme of Noir getting off on light masochism, but it's played more for laughs than for kinks. Things get a little uncomfortable in episode 8, when Noir gets involved with a dryad who takes the form of a prepubescent girl, but for the most part the sexual situations are consensual and take place between characters of similar-looking ages. In a genre space that's been steadily pushing the envelope more and more when it comes to sexual content, it's relieving that we can still have a fanservice-themed anime that knows when to hold back, even if it's a bit too tame for my own tastes.
In the end, if you're looking for a light-hearted ecchi fanservice romp, then Hidden Dungeon is a decent choice. It is by no means a standout in a crowded genre, but if you've taken interest in this anime to begin with, it's probably exactly the kind of thing you're looking for. Despite the lackluster animation, the character designs are cute, and the plot is simple enough that even good production values wouldn't elevate its appeal much further. It's exactly good (or bad) enough to scratch that itch for harem anime comfort food.