i remmeber playing an old strategy game and in that game every city and fort had a kind of "invisible" army in it wich was the garrison so maybe create such a kind of army for every city with the region basic unit of militia could work ? make it like 10% of the population automatically spawns in the city when a batle is started and then disapears once the batlemap ends ?

i mean i don´t know how that could work but it sure added a nice level to the gameplay, also for balance issues those militias shouldn´t appear if the city had recently been taken (in the last 10 turns) or if the attacking faction shares the same culture as the city udner attack and the defenders are from a diferent culture

also these militias shouldn´t contribute to the "garrison" level of a region

if it could be implemented that they only appear in the batlemap it would work great imho but ofc in the automatic resolve batle it would have to be tweaked properly