(Late to the party, but....) I would call it fantasy as well, but probably more so because I've not read much sci-fi to really identify the split. I think the real point is that Chris does a great job of skirting the boundary between sci-fi and fantasy and makes it work. He does this well in the Ketty Jay books as well.
The world building has been commented on quite a bit, and I'll agree. I again found it similar to the approach he took in the Ketty Jay books. There was so much to explain, with such a different setting to what we are used to, and he managed to portray this through very little infodumping. Very streamlined experience.
I also thought the characterization was great. One of my favorite parts of the book was when one of the twins (I'm pretty sure it was Casta) says something along the lines of "We forgot about you. We do that." Mostly I loved that because it was set up so well, and it is something you expect of them. But I also thought it was great because it is one of the flashbacks, and you know and have a better sense of the twins personalities than Orna does. That is one instance, but I found most of the characters to be well constructed and consistent in that way. It was also this that helped me kind of figure out the ending:
I didn't work out that Casta was the Silverfish, but I did know she was the traitor. And I also figured that Ledo was working on a peace treaty and was not the traitor. The second bit I only figured out 10 pages or so before Orna attacked, but I worked Casta out fairly quickly. From the little detail that we were given of Ledo's personality, I didn't feel that it was in him to betray the Eskarans. And the coldness that Casta had about the war and the certainty that she had that it did not serve the class for it to end, pointed her out to me as the traitor. There were other small things that helped me come to those conclusions, but those were the main points.
But despite this, we also see character growth, even if it is given to us in a somewhat unconventional way. We see Orna as she is now, and she doesn't really sway in her personality or develop in any way, but her back story reveals much of the growth that she has gone through and kind of reveals how she comes to the conclusions that she does, despite the fact that they are so far from being right, which must have been exceptionally difficult for her to process, given that its her job and lifestyle to find out the truth. She jumped to conclusions without being sure, which IS out of character, but we are given the back story as to why she does this, and it's made to be believable.
I thought it was an excellent read, despite the fact that I managed to work out the ending. This is something I hardly ever do, and I think figuring it out beforehand kind of dampened it for me. But the journey, and especially the world building and character development, are what made it so special.