I love this game and I'm gonna tell you why, but I'm going to advise you first to go into this one blind. It's going to really make the first leg of the story that much more enjoyable for you.
Anyway, this game is fantastic. The main draws to this game that I want to write about are its story, the characters, and the world-building. Gonna tackle them 1 by 1. There's a lot more I could write about but I'm gonna limit myself to these because wow this review is already long.
The game is one of those slow burners like the dev message tells you it is. You're likely familiar with how these go: character relationships are built up over several hours and the main enjoyment isn't really about the sex payoff at the "end" of the build-up (it is there though, but not for every girl yet as of current update 0.28). You're playing the game to see the characters develop up to bedding them, and then seeing how things progress afterwards. This is not one of those harem games where you go through a 20-minute character arc with a love interest and then they end up just being a trophy you can smash on the side while you pursue the next one.
About those characters then, there's the typical selection of "archetypes" you'd find in a lot of harem games: various shades of shy girls, a couple of flirty girls, a quiet one, a mysterious one, etc. Where Crimson high will impress you is in how it gives you glimpses into the depth of each of these girls. A lot of showing and not telling, done through the small details in scenes like how the girls look at the MC, what they say to him, and how, and sometimes why. 10 hours in and you're still not handed a single one of these girl's full stories and the mysteries of those stories gives each girl a real and distinctive weight whenever they're on screen. The game has probably the most robust and most loveable roster of LIs in any harem game I've played, which is remarkable considering how many LIs there are.
I would talk about the plot here, but it's worth finding that one out yourself, so I'll say it's definitely compelling. Moreover, it allows the characters to surprise you with how they end up developing and interacting with it. A case where both the plot, and the characters are in symbiosis, both enhancing the other, which is absolutely something most games don't get right and where the dev deserves the most praise for managing.
I also want to talk about the MC's character, which a few reviews seem to have issues with. I think they underrate him. He's a detective, and definitely not a dumb one either as shown by how he interacts with what's going on in the plot, picking up on the nuances of what people say to him and being able to discern who he can trust and what people's intentions are. While he's got the typical harem master trait of wanting to take care of and help ALL the girls, he's not a pushover. He'll put his foot down when it's time to do so. He's a 24 yr old adult and he's dealing with a bunch of 18/19 yr old girls at the end of the day, and it's always apparent. If you give him a chance and pay a bit more attention to his monologuing and actions, he will impress you with his insight, or at the very least make you respect him as a character who's able to take the weight of so many people's lives and the responsibilities that come with them. Finally, just like the girls, there's a lot we/he does not know about himself. He has many inexplicable reactions to things, able to do things and react to situations much more competently than anyone expects, all of which in an interesting and grounded way. He has a real depth there, not at all like the typical plank or blank page but somehow competent MC.
Last is world-building. It's done pretty damn well. The game is based in the "real world", in Japan, but here are characters from all over the world, people who are way older than they appear, the threads of politics and intrigue are not explicitly laid out bare the story yet, but absolutely pulse and set the course of things. Again, depth, there's a lot of it and can hold the crazy plot and its fantastic characters in it and keep everything grounded with the rules and expectations that it establishes.
This is where my main criticism of the game lies though. Unfortunately, this game has one of the bad harem game tropes where the MC is the only man shown in the story who isn't dead, an asshole, an ASSHOLE, elderly, a dad, gay, or isn't the one and only buddy/bro trope character who is not at all a threat to your relationships. If you realize that there are basically no other "normal" guys in the story, it can put a dampener on the world-building side of things, specifically how it keeps the story grounded and believable in the context of everything. Even just showing an interaction with another normal couple, or better yet have some more cool, likable, and interesting male characters tangentially related to the plot would do absolute wonders. At the current patch point of the story, the MC is the world to most of the girls, there is no harm in letting them interact with another cool genuinely likable guy, who while isn't going try some ntr shenanigans, is again a normal dude who isn't conveniently disallowed from being liked by the girls. Besides making the world more believable, it would be a chance to show the depth (my favorite word apparently) of the girls' feelings for the MC.
Thankfully, this kind of thing can be done at any point in the story, it is a case of literally just adding a new side character. If you're reading this dev I hope I made my case well enough for you to consider doing something about this at least, this game is close to perfect for me and this trope is lame frankly.
4.5/5 game, rounded up because I think most people don't have as big of an issue as I have with not enough male characters in harem, story focused games. One of the best VNs I've played.