Archaeologists have used the term defense-in-depth for decades to describe obstacles erected to thwart attacks. At Dun Aengus, the most spectacular layer of defense was a band of chevaux-de-frise that made passage by horses impossible and passage by foot unlikely. Today, several firewalls might equal several layers of security, but that’s only one kind of defense repeated. Bronze Age architects made sure different tools and skills would be required at every stop to slow down an attack.”]
Source: https://www.csoonline.com/article/2118779/bronze-age-lessons–practice-defense-in-depth.html