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
Battle of Cannae intercalations, either by accident or for political advantage. A review of the evidence led P.S. Derow to identify the equivalent Julian date would be 1 July 216 BC; other authorities have suggested other Julian dates. Figures for troops involved in ancient battles are often unreliable, and Cannae is no exception. They should be treated with caution, especially those for the Carthaginian side. The Carthaginian army was a combination of warriors from numerous regions, and may have numbered between 40,000-50,000. Along with the core of an estimated 8,000 Libyans, there were 8,000 Iberians, 16,000 Gauls (8,000 were left at camp the