"The Candy Castle" is the second full story in the game. It is made up of the following chapters...:
Chapter 5 - Eat and be eaten
Chapter 5 - Conclusion - The Pied Piper (Pied Piper boss fight)
Chapter 6 - You are what you eat
Chapter 7 - Follow the Crumbs
Chapter 7 - Conclusion - The Candy Witch (Candy Witch boss fight)
Head through all these parts and the achievement unlocks during the summary after the boss fight.