The walled city concept died out the day the English discovered you could fire cannons at city walls and knock a hole in them.
Or was that the Turks, or the Chinese, or whoever!!!!
Today light infantry carry weapons with them that can knock down any wall you want.
BTW, the unfettered access to town walls the cannon had ENDED the day Jeanne d'Arc was told by "the voices" to aim your own cannons at the other cannons.
HIstory was never the same after that!
I'm not a fan of the bunker mentality. It always assumes that when TSHTF that we all go back to 100AD. The reality is that at worst we'll go back to about 1850 with 1930 being even more likely due to the infrastructure that's in place and will hold for several years.
I drew up a quikie spot map of all the little towns across the southern half of my county. The dark lines are connections I have in other little towns. The terrain is mostly lakes, rivers and swamp with forest and farms on the higher ground.
Defensively, I think I'm better off staying right here. Urban zombies don't organize and would be at the mercy of the locals pretty much everywhere they went off the interstate. Anyone who thinks they will be able to withstand an attaack by our military is simply fooling themselves.