How the mighty have fallen. For hundreds of years, the seemingly invulnerable Roman Empire conquered and expanded, eventually sprawling from the Scottish lowlands to Egypt and the Red Sea. Reaping the rich spoils of slavery, pillage, trade and military conquest, Rome took what it wanted. Its borders expanded for centuries. But then it ground to a halt. Rome's centre grew soft. Its leaders became decadent, its armies disillusioned and its outposts neglected. In the forests of northern Europe