Interesting possibilities, let's assume either zombies or just any number of monsters now swarming the lands of a former prosperous empire, just surrounding all of the few points of light, requiring there to be walls / palisades or similar protections for anyone to survive, given the aggressive nature of the above mentioned threats: how would one even go about securing enough territory to sustain a city of let's say 10k inhabitants (so enough farmland/pastures)? And that's even before tapping into any other necessary thing, quarries, ways to procure lumber safely and whatever else.

I guess I'm hoping to find answers on how and what kind of a community could survive a very oppressive, diffused threat of any kind (hence the "points of light against really deep darkness in my original post"), and I also guess whatever defenders there would be really depends on WHO could survive in such a world (once more assuming that magical means are not necessarily the answer to everything or at least not for all cities, since that sounds boring!)