Chapter 2 of Earthopolis: A Biography of Our Urban Planet discusses the crucial role of cities in building empires, defined as states that conquer and rule multiple cities and their hinterlands. It also argues that empires decisively influenced the size and shape of cities. While it acknowledges that camp- and village-dwelling peoples could conquer large empires, it argues that imperial regimes always required capital cities to administer empires over long periods. It illustrates these arguments with references to cities throughout the world: imperial capitals, administrative and monumental structures of many kinds, city walls, provincial capitals, armies and army camps, roads, canals, settler cities, and the infrastructure of imperial borders, including the Great Wall of China and the limes of the Roman Empire.