I agree with some other reviewers here that the characters in this story are under-developed. Vivien is quite likeable and fairly well-delineated, but she reacts in a way that is much less emotional than you'd expect from a child who is suddenly dragged from a rural railway station in a village where she is being evacuated in 1939 to a crumbling city outside time, by two irritating boys, for no really convincing reason that I could see. Jonathon and Sam are both slightly tedious, I thought, though I felt Sam came alive more than Jonathon.
I loved Elio and I thought Sempiturn Walker was a good creation though I got very bored with the long descriptions of his running about in a panic before every ceremony. Though Vivian finds this hysterically funny, it just didn't seem very funny to me - not after the first time, anyway. But this might be because I'm not a child myself.
As with all Wynne Jones's books, this is highly imaginative and original, and I thought the guardians were fabulous inventions - I also liked the way the villains turned out to be who they were as I wasn't expecting it. I like the way Jones is unafraid to present some children as hideous and cruel rather than as all brave and good, and I like the way she doesn't always dismiss adults, like some children's books do, but presents children and adults as equally likely to be good, bad and complex. I thought, as with some of her other books, that the story was padded out slightly around 80% of the way in, in order to make it longer, but the final climactic stretch was pretty well-paced, I thought.
The plot is convoluted and unusual, and there is the trademark sly humour, but this novel didn't involve me as much as some of her others have, and didn't create quite the atmosphere I've come to expect from Wynne Jones. But nevertheless, if you're a fan, you'll enjoy it.