How To Defend Your Urban Perimeter: The Atomic Wall

"Walls have protected cities as long as there have been cities to protect. To guard against invasion, Nebuchadnezzar built a network of brick walls so that 'the evil and the wicked might not oppress Babylon.' As weapons evolved, walls had to change with them. By the 16th century, artillery had advanced to the point that Vienna razed the scattered developments outside city walls so potential invaders would have nothing to hide behind. The battlements had become a broad detection zone. The width of the space created was 400 meters - the range of cannons at the time. The outline of that zone is still visible today in the avenues and parks that make up Vienna's fabled Ringstrasse.Airplanes and missiles rendered the city wall symbolic; these days, the important defensive barriers aren't physical fortifications. But the advent of small nuclear weapons and dirty bombs — deliverable not by missiles and planes but by trucks and vans — suggest a new kind of urban perimeter defense, an atomic wall. Set up not as an actual barrier but as a vast array of sensors, such a technology would exploit the fact that any radiological or nuclear weapon leaves a footprint."

Thanks to Chris Steins