For centuries, Rome governed hundreds of thousands of people in an empire that covered as much as 2 million square miles. In an age when most travel was done on foot, how was this possible? Digging deep into the lives of two emperors who presided over one of the most successful periods in Roman history, NGC explores how the empire owed its longevity to its ambition, innovation and genius for administration. See how Rome endured for centuries as one of the greatest bureaucracies on earth.

• Kieran Bew