Defence in depth in order to retreat successfully, and they assume that the defender's morale will recover from the retreat. A possible early example of this came at the Battle of Cannae in 216 BC, when Hannibal employed this manoeuvre in order to encircle and destroy eight Roman legions, but that is disputed by some historians. Edward Luttwak used the term to describe his theory of the defensive strategy employed by the Late Roman army in the 3rd and 4th centuries AD. Later examples of defence in depth might be European hill forts and the development of concentric castles. In those examples, the
Sanitation in ancient Rome sanitation artifacts of the ancient world. Most sources believe it was built during the reign of the three Etruscan kings in the sixth century BC. This "greatest sewer" of Rome was originally built to drain the low-lying land around the Forum. It is not known how effective the sewers were, especially in removing excrement. From very early times the Romans, in imitation of the Etruscans, built underground channels to drain rainwater that might otherwise wash away precious topsoil, used ditches to drain swamps (such as the Pontine Marshes), and dug subterranean channels to drain marshy areas. Over time, the Romans