To fund his army and defend his crown, William the Conqueror needed lots of money. He decided the best way to get hold of this was to tax people according to the value of their land: a survey of shires. It took a whole year to complete the survey. The information was written down in a book. The book was later called the Domesday Book, because it was thought to be as detailed as the assessment of someone's life that God makes on Judgement Day/Domesday.