Thunder City by Philip Reeve

Tamzin Pook is a fighter in the Amusement Arcade. And what she does best is killing Revenants. All she knows is survival, having arrived in the Arcade as a small child. She doesn’t dare to hope or wish for anything more than to survive another day. Meanwhile, the wheeled city of Motoropolis has been taken over by a rebel faction who killed its leaders and commandeered the city. Its only hope is a teacher named Miss Torpenhow who’s determined to find the Mayor’s good-for-nothing son and force him to take back what’s rightfully his. But to get to him, she’ll need to find someone who’s skilled at fighting Revenants … With a daring abduction, Miss Torpenhow and Tamzin’s destinies are entwined, and so begin theiradventures together.