Do I even need to mention that Marc Platt has a flair for the Gothic? For all that this book is ostensibly a science-fiction novel that involves two timeships colliding and forming a tangle of paradoxes, in all practical terms it's set in a baroque Gothic castle straight out of Gormenghast. Ghosts of people who haven't died yet wander through the novel, strange omens and portents abound, and Gallifrey--Gallifrey, of all places!--is run by a mad soothsayer who curses the planet to eternal sterility. It's more Gothic than 'Ghost Light', and that's saying something.
And as with 'Ghost Light', Platt's previous Doctor Who story, it's really more of a fugue (in the musical sense) than a narrative in the traditional sense. The concept of time being tangled, of space and time being intermingled in strange and disturbing ways, is introduced early and then recapitulated in any number of different ways. Ace finds the same bicycle she's going to pick up in another part of the City, the Pilots' brutal overseers turn out to be their future selves, the Process bickers entertainingly with its older and wiser self in what has to be the most amusingly catty dialogue ever recorded between two dim-witted leeches. The plot almost doesn't matter; what's important is the atmosphere.
Which does mean that it can get to be a bit much, at novel length. The Ancient Gallifrey scenes, which are probably what the book is best remembered for, serve to break up the monotony of the gray and lifeless City when things get to be too oppressive even for a novel designed to be something you inhabit more than read. And they do have some good stuff; the Gallifrey that Platt imagines somehow feels right, a strangely organic outgrowth of the material we've already seen that would actually be interesting even if it weren't the distant past of the Doctor's homeworld. Far too many writers assume that a standard sci-fi trope (aliens invading, evil rulers, traitors to the throne) will somehow be magically made interesting if they add the words, "...on Gallifrey!" at the end. Platt gives us a world that we want to read more about, but never gives us too much. It's no surprise that he was finally selected to tell the closest we'll ever get to the origin story of the Doctor.
Like the best horror stories (and what is a Gothic but the horror story's ancestor?) this lives or dies based on whether the reader can transport themselves into the setting. If you feel the oppressive gloom of the City, if you taste the dry and deadened dust on your lips and picture the twisted towers, you will love this one. If you don't, I suspect it comes off as a brutally dull experience. But "suspect" is the operative word, because this is one of the Doctor Who novels I love.