Within the City Walls is a Speculative Fiction / Young Adult novel.
The world in which they inhabit is one in which the City of London has made itself an island. Economically less than physical, other than the wall that surrounds the parimeter of the City which Noah and the other apprentices are training to defend.
It is speculative fiction in that the City is not seperated from the UK and we don't have Hovercraft transporting workers up and down the Thames from the feeder towns. Outside of this all technology in the novel is based on real life examples.