“The Castle didn’t seem to care if you were descended from a royal line, or if you were brave or intelligent. No, Castle Glower picked kings based on some other criteria all its own.”

This quote reveals that the castle can do more than simply rearrange itself periodically. The fact that the castle chooses who will rule the kingdom presupposes not merely sentience but also intelligence and judgment. Furthermore, the reigning king submits to that judgment.

“Celie had kept the master copy to add to her own atlas, and on her way to her room she began to muse about how many pages it might be when it was finally completed. But how could she ever finish? The Castle grew new rooms every week.”

Celie’s pet project is the completion of the atlas. At one point, she even fantasizes that finishing the book will bring her parents back. This quote suggests that her fondest hopes may never be realized because the castle will always continue to expand.

“Celie just goggled at Rolf for a minute or two. She could no more ask the Castle to provide rooms for the princes than stand on her head on the roof of the Spyglass Tower!”

At this stage of the story, Celie does little more than record the castle’s layout changes. It never occurs to her that her understanding of the physical structure may indicate a close personal tie to the essence of the castle. Rolf’s suggestion spurs her to make a first attempt at direct communication.