The King's Secret. The Zero Enigma, #13

For centuries, the Kingdoms of Tintagel and Garstang have been rivals, battling endlessly over the borderlands with neither kingdom able to win a decisive victory, let alone knock the other out once and for all. But now, Garstang has unlocked the secret of recreating ancient war machines and unleashed an army out of nightmares, a magical-mechanical force grinding its way through once-impregnable defences and advancing on Shallot.
Giant metal monsters stalk the land, crawling metal locusts tear through walls and defences, flying machines hover overhead or roam far behind the lines. And nothing can stand in their way. For Lucy Lamplighter, disgraced and sent into de facto after the civil unrest in Shallot, the war represents a chance to redeem herself, even at extreme risk to her life. For Adam Mortimer, reeling after the events in Kirkhaven, the war represents a challenge that can neither be denied nor won.
But as both monarchs reveal their secrets, and newer weapons of war take the field, devastating the land around them, it becomes clear that the price of victory will be terrifyingly high .And in the shadows, the real threat is slowly inching into the light.