The Bridge. The Bracken Series, #1

Princess Rosalynn is pretty, spoiled, and childish. She gives no thought to the men and women that protect her father's throne. Then one day, invaders from the enormous kingdom across the river storm across the Bridge and conquer the royal castle. Alone and in danger, Rosalynn sets out on foot to find her father. Driven by fear, she finds the Bridge and crosses it. In the enemy kingdom, she meets an unexpected friend--a woman who has discovered a secret way to save Rosalynn's kingdom.
Together Rosalynn and the wise woman face enemy soldiers, wicked outlaws, and the wilds of the river on their quest to save her tiny kingdom. Their goal is the Bridge itself and the secret that it holds.