Path of Beasts by Lian Tanner (Final book in The Keepers Trilogy).
Summary: Do you dare to join Goldie in her hardest task ever as she walks down the mysterious Beast Road, deep inside the Museum of Thieves? No one knows where it goes and no one has ever returned from it.
The city of Jewel is in peril once again, as it is held captive by the frightful Fugleman, his band of Blessed Guardians, and an army of merciless mercenaries. There's no doubt that Goldie and Toadspit want to get their city back, but how can a small group of children fight against such overwhelming forces of evil? And how, as Goldie is determined, can they avoid bloodshed in a war that will set thieves against soldiers, and trickery and deception against a mighty cannon that shoots cannonballs bent on destruction? As Toadspit fights the Fugleman in a duel to the death, Goldie must face her hardest task yet. If she is to save the city, she must walk the mysterious Beast Road, deep inside the Museum of Thieves. No one knows where it goes and no one has ever returned from it.
(Published: October 2012   Updated: October 2012 –Books In Print)