What chases Milo, Tock, and Humbug up the stairway to the Castle-in-the-Air?

Answer: Demons

Racing ahead, the three companions soon come upon the staircase to the Castle in the Air. As they reach the first step, they notice a little man at a desk with a huge book upon it. The Senses Taker asks all sorts of questions about the travelers that become increasingly trivial, and he writes down all of the answers in his ledger. They become engrossed in the question game and forget all about the castle and the horde of demons chasing them until Milo drops the bag of sounds that the Soundkeeper gave him. The package breaks open and explodes with laughter, pulling Milo from his trance since humor is the one sense the Senses Taker cannot seize.