Eternity Engine by Struan Murray

The final instalment in the breathtaking Orphans of the Tide trilogy which The Times calls Unputdownable. The seas have parted to reveal the remains of humanitys lost empire and opened a path from Shipwreck Island to the Enemys City. Now only Ellie Lancaster and her orphan friends can prevent a terrible war. As Kate marches north at the head of her army panic infests the City as the Enemys strength grows. For the parting of the seas has also uncovered the Eternity Engine a dreadful machine from Paperback softback