Baxter has long since established himself as having a great talent of speculation, and he demonstrates here that he's just as good at doing it in the stone age as he is in the distant future.
The central idea behind this story is the building of a wall to hold back the rising seas, which overhwhelmed low-lying land between England and Europe in the real world. What we have is a compelling story about how this affects the main characters, and the conflicts that result. The characters of this novel are one of its strengths, and demonstrate Baxter's continuing trend of writing more complex and believable characters in recent books. Few of the main characters here are completely likeable, and the audience is lead to empathise with different characters at different stages of the story.
There are two reasons why this doesn't get a 5. The first is the considerable number of implausibilities in the story. I seriously doubt that the hunter-gatherers described at the start of the novel would have got anywhere building the wall as it's described, and the likelihood of mother and baby surviving an amateur Cesarean section is also pretty low. Finally, the pseduo-human tree-dwelling "leafy boys" are also pretty ridiculous. They seem to be an attempt at describing stone age aliens. And like stone age aliens, they don't really work.
The other problem in my view is the wall, or rather the lack thereof. I was quite surprised at how little of it we actually saw. Once the idea of the wall is hatched, it serves as more of a backdrop and a motivator for the rest of the story. The novel might be better termed "a story about the conflict that took place while the wall was being built", rather than a story about the wall.
What story Baxter does tell, however, he tells well. I look forward to seeing where he takes the concept next time.