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
Titus Manlius Torquatus (consul 235 BC) to cross the Po river, then defeated the Boii there. The Celts lost 23,000 men and 5,000 of them were captured, but the consuls had to retreat because of poor weather and decease among their troops. In 216 BC, as one of the most senior senators then living, he successfully opposed the ransoming of the Romans taken prisoner at the Battle of Cannae, on the grounds that they had made no effort to break out of the Carthaginian lines. In 215 BC he was sent to Sardinia, after the illness of Quintus Mucius Scaevola and defeated a Carthaginian attempt under