Even though we may not build overt, physical defense mechanisms into our cities, our cities are still in many ways engineered for defense.
Drones, SWAT, surveillance, checkpoints, security zones - all of these techniques are employed, from Baghdad to New York, to exert control. So while we don't build walls to keep people out anymore, we certainly still build defenses to protect us from the people already inside the city.
What I disagree with the author on is scale - all of these countermeasures are still on a larger scale - it makes more sense for a city government to enact safeguards, than it does for individual buildings. There really hasn't been a disappearance of large, city-wide protection systems, you just can't see them as obviously.
Very nice, but http://en.m.wikipedia.org/wiki/Palmanova beats both in the symmetry department (rounder than Naarden. Also note the symmetrical outgoing roads on the 17th century map)
Not only can you see where the fortifications of Vienna were, you can even make out the rectangular structure of Vindobona, the old Roman military camp in the city's inner district:
