Moated Manhattan.(The Talk of the Town)(moats and trenches)

Manhattan already has a moat--the rivers Harlem, Hudson, and East--but this on its own will not suffice, at least against the incursions of would-be terrorists and unwelcome drivers. And so the city's defenders, in recognition of these dangerous and congested times, must seek other means of fortification. After the attacks of September 11, 2001, they experimented with planters and bollards--those bulky barriers guarding skyscrapers--but last fall began removing many of them, because they were redundant, weaponizable, disruptive of the pedestrian flow, or useful only as ashtrays.

Recently, other plans have come to light, which together suggest the creation of a kind of electronic super-moat. Already under way is an effort to place, around the city, radiation monitors, which will alert authorities to any radioactive materials that might be in the vicinity. And in recent months the Police Department has been developing a scheme, inspired by London's so-called Ring of Steel, to set up a web of thousands of surveillance cameras around town, starting in lower Manhattan. The police will be able to deploy remote-control swinging gates--latter-day portcullises--to block streets, should the cameras suggest that something sinister is afoot. Finally, there is Mayor Bloomberg's effort to introduce congestion pricing. Under the system, drivers entering or leaving any part of Manhattan south of Eighty-sixth Street during certain hours will have to pay for the privilege. This will require the installation of a ring of sensors and cameras, which will be capable of photographing license-plate numbers, for the purpose of accurate billing.

For centuries, cities have been gradually opening themselves up, shedding their walls and sprawling outward, so that their boundaries are ever more porous and blurred. (One great catalyst for this was the automobile; another was Napoleon, who decreed that walls surrounding Italian cities be taken down.) So it is notable, and perhaps inauspicious, that a reversion to a distinctly medieval "Who goes there?" approach is in full swing.

According to Mark Munn, a historian of ancient Greece at Penn State, the Roman Empire began erecting fixed barriers, or limes, along its ...