…and there he corrected many abuses and was the first to construct a wall, eighty miles in length, which was to separate the barbarians from the Romans.
— Cassius Dio, second-century Roman historian
The Wall is the Wall, it has never changed or evolved from the first day I conceived of it.
—Donald J. Trump, 45th President of the United States
Two and a half thousand years after its construction, the fortifications at Dún Aonghasa are an alien artifact — desolate rings of weather-beaten stone laid out on a windswept cliff on Ireland’s Aran Islands in the North Atlantic. But upon closer examination, Dún Aonghasa betrays sophisticated and familiar strategies of defense. These concentric walls and ditches built five centuries before the birth of Christ are flanked by a tangle of limestone stumps that serve as anti-cavalry measures called cheval de frise. It’s a location that offered a good vantage point and a safe refuge for those who held it. Dún Aonghasa presents early evidence of the same principles of redundant security measures at work in 13th century castles, 17th century star-shaped artillery fortifications, and even “defense in depth” security architecture promoted today by the National Institute of Standards and Technology, the Nuclear Regulatory Commission, and countless other security organizations world-wide.