History seems to indicate that in the absence of law, those with the most people inside a fortified structure and position are the most likely to survive.

I don't think that's true. I imagine the people with the highest chance of survival are the ones whose governing/ruling people seek peace and the rule law quickest. Second would be people who flee to the nearest safe and lawful area. A fortification is probably the third best option if you can't have either of the first, but the probability of that structure keeping you alive is very low, especially if the conflict lasts long enough to become a siege. Entire cities managed to hold out from sieges that lasted for years, but the ordinary people inside did not.