Complex sentence: 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.
Split into simpler sentences: 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. Even worse, they will be in his hometown of London, where he is at his strongest.