What is the major problem in Tuesdays at the castle?

In "Tuesdays at the Castle" by Jessica Day George, the major problem arises when the castle's magical abilities to change and adapt are put to the test as its inhabitants face external threats. The young princess, Celie, must navigate the challenges of protecting her home and her family from intruders who seek to take control of the castle. As she learns to harness the castle's magic and form alliances, the stakes grow higher, forcing her to confront her fears and prove her resilience. Ultimately, the story highlights themes of bravery, family, and the importance of believing in oneself.