The people fed on the magic and the magic fed on them until it ate their bodies and their minds and then their souls.
-- Kell (Part 1, Chapter 2 paragraph 3)

Importance: The four Londons used to be accessible to anyone through magical doors, but, as Kell explains here, Black London was unable to exercise restraint when using magic and the situation there escalated out of control. This necessitated the sealing of the doors so that only Antari can pass from world to world now using blood magic, though even they stay away from Black London. This is one of the major themes of the novel, that magic is dangerous and requires caution from its practitioners.

I feel more like a possession than a prince.
-- Kell (Part 2, Chapter 3 paragraph 6)

Importance: Kell recalls arguing with Prince Rhy about his status in the royal family. Kell feels like he is not a true prince and that he is only valued because of his magical abilities...