What is the importance of The Tower of London in the novel, The Mirror and the Light?

An infamous prison, the Tower of London becomes Cromwell's home away from home in more than one way. At the beginning of the book, he's so familiar with the Tower that he becomes godfather to the head jailer's daughter; by the end, he's a prisoner there himself. The Tower is a place of curious contrasts: elegant apartments for housing noble prisoners share a roof with instruments of torture. When Cromwell is at last imprisoned, he feels the Tower haunted by the innumerable ghosts of those he destroyed and those he tried to save.