What is the theme in Castle in the Air by Diana Wynne Jones?

Fate is one of the main themes in "Castle in the Air" by Diana Wynne Jones. The fortune teller predicts that Abdullah's fate is sealed from the moment of his birth.

"These are the words of the wise fortuneteller: 'This son of yours will not follow you in your trade. Two years after your death, while he is still a very young man, he will be raised above all others in this land. As Fate decrees it, so I have spoken."