Archaeologists have used the term defense-in-depth for decades to describe the obstacles erected to thwart attacks. At Dun Aengus, the most spectacular layer of defense was a band of chevaux-de-friseupturned stones jutting in every direction that made passage by horses impossible and passage by foot unlikelythat completely surrounded the middle and inner enclosures. But it's instructive that this is just one of several kinds of defenses at the fort.Viking marauders running up a hill to take a fort would have to survive a series of defenses, arrayed in sequence: berms, ditches, outer walls, chevaux-de-frise, more ditches, walls, palisades (tall, spiky wooden fences) and more walls.Infosecurity professionals practice some defense-in-depth, but a key lesson from Dun Aengus is the variety of defenses. 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 and therefore improve the ability to counterattack.