Maybe the solution is to locate them in countries that are sufficiently weak militarily that they cannot take over the charter cities easily. A lot of places cu

Maybe the solution is to locate them in countries that are sufficiently weak militarily that they cannot take over the charter cities easily. A lot of places currently meet that criterion. Medieval cities had walls which were generally enough to keep the feudal overlords out. They were individually quite weak militarily. In Italy at least they succumbed quickly to foreign powers like France after 1494. It's probably bigger outside powers you want to worry about.