Final answer:
When Agatha clung to the gutters of the School of Evil tower, something terrible happened that has haunted the house ever since. The thought in Lucy Dare's mind during those hours while she made her choice created an unseen and spiritual environment that was more real than the material fact of the house. This unseen environment has left an indelible impression on the things that surrounded Agatha.
Step-by-step explanation:
When Agatha clung to the gutters of the School of Evil tower, something terrible happened that has haunted the house ever since. The thought in Lucy Dare's mind during those hours while she made her choice created an unseen and spiritual environment that was more real than the material fact of the house. This unseen environment has left an indelible impression on the things that surrounded Agatha.