I loved Sara Douglass's earlier novels, especially the six that comprise the Wayfarer Redemption series. The "Darkglass Mountain" series is a continuation of that saga, with a few of the characters returning. "The Twisted Citadel" and its predecessor, "The Serpent Bride," are certainly better than the "Troy Game" series (which started well, but I found quite tedious by Book 3), and the dreadful "Crucible" trilogy, which I couldn't finish.
To give Douglass her due, she is very good at creating a vivid fantasy world, peopled with interesting and often bizarre characters. Her plotting is intricate (I won't attempt to summarise it here) and generally holds the attention, though I confess I found "The Twisted Citadel" quite slow moving at times, and I had a sense of deja vu in respect of the plot, too: eg. Maximilian's distrust of Ishbel parallels Axis's attitude to Azhure in the first of the "Axis" books, although most likely this was deliberate. Douglass's flair for description is very much evident, and she certainly doesn't spare the reader during the darker moments of the novel.
What bothered me about "The Twisted Citadel" though was the quality of the prose, which is unfortunately a bit patchy. I am not sure whether the writing style has deteriorated, or whether it was always like that and I just notice it more than when I first started reading her novels. Whatever the case, there is a lot of redundant or overly expository dialogue, and the characters have an irritating habit of repeating the name of the person to whom they are speaking in almost *every* line, so you start to wonder whether they are all telemarketers in their spare time: I was half hoping Ishbel would eventually say to Maximilian, "I know my own name, thanks - you don't have to remind me of it every ten seconds." I also found parts of the narrative a bit repetitive, and could have been tightened up somewhat. The characterisation also needs a bit of work; the major protagonists - Ishbel, Maximilian, Isaiah, Axis etc - are portrayed in a way that renders them rather remote, and as a result while I was objectively interested in what would happen to them, I did not feel any real sense of attachment to them.
Nonetheless, it did keep me reading, and wanting to find out the ultimate conclusion in the third book. Overall, therefore, it's okay, and warrants a solid three stars. It's certainly essential to have read the prequel first - this book is not written in a way that someone new to the series could totally make sense of it as a stand alone novel. I'd also recommend that readers seek out the Wayfarer Redemption books first and read them chronologically, because the references in this and "Serpent's Bride" make much more sense if one has read the preceding series; in addition, some of the central plot points and themes of this novel hark back to an earlier stand-alone one, "Threshold."