Next was a castle divided into many small rooms, with a system for passing 
messages between rooms through a pneumatic tube. In each room was a group of 
people who responded to the messages by following certain rules laid out in 
books, which usually entailed sending more messages to other rooms. After 
familiarizing herself with some of these rule-books and
establishing that the castle was another Turing machine, Princess Nell fixed a 
problem in the message-delivery system that had been created by the vexatious 
dark knight, collected another ducal coronet, and moved on to castle number 
six.