If Gilgamesh refuses to teach the twins they will be unable to escape back to San Francisco using the Ley Lines, and will be trapped in Dr. John Dee's city, his hometown of London, where he is at his strongest.
They will be unable to escape back to San Francisco. This San Francisco is using the Ley Lines. They will be trapped in Dr. John Dee's city. Gilgamesh refuses to teach the twins. Dr. John Dee's city is his hometown of London. He is at his strongest.