The Siege of Macindaw (Ranger's Apprentice Book 6)

After years as a Ranger's Apprentice, Will is now the protector of his first fief. Not long into his service, everything that can go wrong, does: Keren, a renegade knight, has taken over Castle Macindaw, a strategic gateway to the North - poisoning the royal family in the process - and is holding Will's friend Alyss captive.