If there’s something that this manga has done perfectly, it's in creating atmosphere. There’s nothing new to be said about a post-apocalyptic world where machines have outlived humans in a sprawling urban zoo, but the minimal world building verbally expressed compared with the extensive environment and atmosphere shown through Yuu and Chi’s experiences creates a strong bond between the reader and main character. There's a beautiful juxtaposition of us viewing this depraved world through the eyes of someone who has adjusted to this way of life, and them experiencing relics of the past we may be used to with regular fascination.
I love massive magnitudes, and the constant need for them to be travelling upwards through these towering structures creates a really beautiful sense of scale that only amplifies the emotional impact that comes with being some of the last humans alive. Naturally, that overwhelming presence gives the close bond between the girls a beautiful backdrop to be compared to. We experience this warm and loving bond against a massive, soulless environment: reading books together atop a multi-story bookcase in a tomb-esque room, snowball fights on top of an oppressive tower that stands above anything else, baths in water tanks that used to feed humanity, long since empty. It’s a story about perseverance of character and enjoying what life has left to offer in a place that hasn’t felt warmth in decades.
If there are issues for me to bring up, it would be that it’s never expressed to us why the world has ended up this way, even subtly. I love the minimal world building as I’ve said, and how instead the focus is on the emotional impact of the environment. However, in the estimated 10 or so years between when we know there was cohesive society and the current events of the manga, something has happened which has wiped out most of the human and animal races, along with massive amounts of infrastructure. I would find this point understandable if it had happened before the girls were born as to detach the reader from any connection with the past world through the main characters, yet it’s strange to me that the girl’s have clearly experienced the downfall of civilization firsthand and that’s never explored or mentioned beyond them being sent out by their grandfather when they were very young.. At first, I felt the atmosphere implied that humanity has been strongly declining over decades. I was surprised to learn how quickly it all happened and how that fact is only glossed over. If anything it would make more sense to me that the girls have always experienced this type of life.
One last subtle point: for being some of the last humans alive, when Yuu and Chi meet other humans, there really isn't as much excitement as you'd expect? I think it's fitting with the tone of the story, and for that I wouldn't change it, especially when the interactions between the characters continue to be beautiful. Just something I noted.