It was relatively good until the movie reached its pre-climax, where it broke most of its own rules, which were established before that point. Can't say it was "amazing" by any stretch, it was slow, there was very little progression, and most of it was predictable. Being predictable is not necessarily a bad thing: I've personally seen more than enough stories, that are predictable, but are still told in such a way, that keep you glued to the screen. It seemed to be one of those, as we were slowly learning more about Kokoro's trauma, and as she the kids in general were getting closer to each other, but they ruined it. Sadly, to explain how exactly the movie was ruined, I need to get into heavy spoilers.
Slowness is not a sin
One of the complaints can be that it was still too slow, but it may have been part of the story: (almost) all of the kids seem to have some level of social anxiety, so it would be understandable that they would need time to open up to each other. Even Wolf Queen mentioned several times, how they were slow at everything. On the other hand the story was focusing on Kokoro, so it was not clear how the other kids play into all this besides introducing a sense of belonging to Kokoro.
That is until the pre-climax, when they dumped histories of all other kids into Kokoro's mind and onto our screens. This felt weak, as if the authors were making up this story, focused on Kokoro, and then suddenly realized: "Oh, we have like 15 minutes left, and we did not cover any other characters, we need to deal with this ASAP". The kids already felt mostly unnecessary, but this did not elevate them in any way, but rather made things worse. With 1 character in particular.
Hidden importance
During most of the movie we had a relatively calm and light atmosphere. We did not feel any danger or any darkness even. And then suddenly we see a father (I assume, it was not really clear) to sexually abuse Aki, one of the kids. This is a serious topic, like really serious, and the way it was added felt extremely superficial. Yes, there was a prior moment, that kind of hinted that there may be some abuse involved, but that was it. It felt like the topic was downplayed, that it was there just for the sake of adding "an edge". Which is even more of a problem considering next point.
Aki turned out to be a very important character for, at least, 3 of the kids (including Kokoro). She is their teacher from the "alt school". There is the time travel kind of nonsense involved here, which does make some sense compared to other stuff. "Hiding" a crucial character like that is nothing new, of course, but usually when it's done the past version of the character has at least some elements, that could link one to their future self. Aki the teacher seemed caring and compassionate, but her past self did not look like that at all. If anything, I would say she was a bit standoffish. If you think her experience of abuse changed her - no, it does not make sense, and I will get to this, too.
No rules rule
The castle was created by dying sister of one of the kids (Rion), and she is also the Wolf Queen. From what I understood she was 7, and seemingly that is the reason why she "summoned" 7 other people, from different years with 7 years difference between each year for each person. Well, almost: it established the rule and broke it right away, because the year of the sister died is included in calculations, so there is already, technically, a gap, and then Rion and Aki are from same year, meaning 1 year is repeated. It's next to random now. All the kids are also summoned from Japan, but... Rion is not in Japan, he is in Hawaii. They are not going to school, except Rion does go to school. They seem to start to establish rules behind the castle, and they flush them the next moment.
Speaking of rules: the main rule of the castle is that you need to get out before certain time, so that a wolf does not eat you. If you don't - wolf will also eat those who visited the castle that day. That already does not make sense, because why would others be punished as well? Makes even less sense, if we assume that the castle was supposed to become a place of healing and a refuge. But then the creator of the castle essentially forces this rule into play.
Aki's grandma died. I guess she was mother of her father, and her father was grieving. He comes up to Aki's room with a face that screams at us "I want to have fun with my daughter". You know the kind of fun, which is not really fun. It did not feel like there was any build-up to it, but ok. Aki escapes, and trying to get out, when she sees a random mirror (and mirrors are portals here), which suck her into the castle. It was the Wolf Queen, who saved her. Knowing that she would die because she would not go back home, unless someone made a wish.
Wish upon a key
So, everyone besides Kokoro ends up eaten by the wolf. D. E. A. D. The only solution is for Kokoro to make a wish, but for that she needs to find the key. Which is a huge "if". Which also makes you wonder if Wolf Queen really "saved" Aki, since she did admit earlier that she is not in control (which was proven wrong, and I will get to that, too) and does not know what will happen.
Kokoro needs to solve a puzzle, which... Does not make much sense. Like, ok, she accidentally found a painting in her friend's house, that looked similar to the interior of he castle and was depiction of "Seven Little Goats" or whatever, which made sense of the weird "X" marks the kids found all over. Ok, I'll buy that, but how did she figure out what she needed to do (specifically, to say) near each of those crosses? And why were they there in the first place? Wolf Queen said they were grave markings of the other kids, but the marks were there before the wolf ate them all. But, ok, this is a fairy tale, so let it be.
Kokoro makes a wish: "Make it so Aki does not stay after allowed time". Great. So what exactly changed in the timeline? Did Aki not go into the mirror, and was caught by her father? Does not sound like a happy ending, even if she became a councilor of sorts. Or did her grandma not die? Or her father changed somehow? How exactly was she saved? And why was it ok to break the rule of "everyone will forget the castle" for Rion? The rule that the Wolf Queen herself implemented? Aaaaaaa!
Wasted potential
It could have been so much better. It could have been a TV series, where we would have at least 1 episode to cover real life of each character, and what troubles them, or what is supposed to start troubling them at some point. Well, whatever happens that made them eligible to be summoned into the castle. Make the castle a place of healing, if possible, but at least make the rules stick and generally make mor sense. Add a clear connection between the characters in real life.
As is, in the end it felt like wasted time. It has so much potential to be a beautiful fairy tale with a strong message (or maybe a couple of those even), but ended up being a total mess, that lands nowhere. To me it caused more frustration than some bad movies I've seen in recent years. I would not recommend it.