Catton is trying to mimic the style and diction of the Victorian novel, and she does it very well. Her language definitely evokes the crispness and propriety of Victorian prose—and speaking of propriety, we should mention that curses get the literary version of the bleep button.
Also, she uses antiquated spellings—such as "gaol" for "jail," "phial" for "vial," and "connexion" for "connection"—that definitely create a sense of Victorian authenticity.
There are 800+ pages of examples, but we'll just pick one that popped out at us because it 1) it uses one of those antiquated spellings and 2) it has a kind of buttoned-up writing style that seems more 1866 than 2013. The example comes right after Moody has met and formed an opinion about Alistair Lauderback. The reflections here are kind of Austen-esque, in that they take Lauderback down in a way that is both mild and devastating all at once:
Moody listened to him politely, but the impression he formed was an unfavorable one, and he had left the scene of their first acquaintance with no intention of repeating it. He saw that Lauderback was the kind of man who did not care to court the good opinion of any man whose connexions could not benefit his own. (II.4.28)
So, there you have it. Even though it was written in 2013, Catton effectively mimics the writing style of the Victorian novel—which is appropriate, given that it's set smack dab in the middle of that form's heyday.