Soon after Polo returned from his journeys he fought in a war against Genoa, got captured and put in prison. When he was in prison, he became friendly with a fellow prisoner, Rusticello, who was a writer of romances and novels. He told the writer about all his adventures, which became a book called "The Travels of Marco Polo". The published version was written by Rustichello da Pisa, based on what Polo had told him. It became famous throughout Europe. In the book he said that Kublai Khan's wealthy new empire had a postal system. He also talked about the Chinese people. China used paper money that was made from mulberry bark.