[SuperLuminal]

"A few insurgents can bring a city to it's knees in a week, let alone what an army of them can do."

Yep! Like, over past centuries, you surround and cut off the castle/city from food and water supplies...Will save lots of ammo...Cities/castles like NYC would self-destruct in a couple of weeks...

[DiogenesLamp]

Modern cities are far less survivable than medieval cities. There are far too many people dependent upon the concentrated infrastructure cities need nowadays. The people are also less willing to tolerate discomfort than were medieval people.