Oh man, oh man, how to describe my feelings about this book?  This amazing and fascinating novel.
NGL, the main thing that drew me to this book was the main characters were pre-established as gay. None of that weird falling in love while insisting they're straight or the whole "I'm only gay for you" nonsense, which is such a relief, I can't even. This book also managed to hit one of my favorite things in writing which is a slow burn romance that is quite literally SLOW BURN. The UST is real and it was a wonderfully agonizing wait for the two characters to get together.
The really great thing though is that the romance wasn't the main point of the story. It was icing on the cake to a fascinating story about two incredibly smart people trying to figure out how to survive in a world that is actively, maliciously trying to kill them. All of the supporting characters were wonderfully developed and just because Tang Mo and Fu Wenduo left them behind, didn't mean that their contributions to the story ended. There were moments when they would go to fight something themselves that left me on the edge of my seat and overwhelmed with anxiety at the thought that they might die. Looking back on it, I kind of find myself marveling over how delicately the bits of character building were interwoven in the story to make me care about a large cast of characters who essentially popped in and out of the story. It isn't that EO didn't have it's share of cannon fodder characters, but I was taken by surprise by the characters I couldn't wait to die only for their deaths to hit me like a punch in the chest (view spoiler) to characters who I expected to hate but ended up becoming terribly precious to me (view spoiler).
In terms of the world building, there were a lot of times while I was reading this when the scenes would suddenly play out so vividly in my mind that all I could think was "I want to watch this so bad" and this is still true. If it was announced tomorrow that there was going to be a donghua, I would be screaming in agonized frustration over the wait until it releases. It dips into things I've seen done before in a way that manages to make it very real to me but also still maintains a sense of fascinating originality. The puzzles were very well plotted out although really difficult to come to a solution on your own. I eventually stopped even reading the rules when they were initially announced because I knew I wasn't going to be able to figure them out, however when Tang Mo and/or Fu Wenduo do, it left me a little astounded.
I think the only drawback for me was that I found the truth of the black towers to be both baffling and anticlimatic and also, there were a few times when I wished the story would just hurry up a little bit. When I finished it, I wasn't thinking man that was really damned good, but I did think it while I was reading it, and after I've really let what I read percolate in my mind, I think it's definitely one of the best books I've read.