ill make this quick and to the point:

Implement bigger population drops for recruiting units, so city populations can become depleted through excessive recruitment.


There is probably a hardcode in the way here, but maybe theres a way around it with clever scripting? Example: When you got recruit a unit of 80 spearmen, the game actually creates a 200 man group but the unit is 'switched' for a 80 man group before the user can use it.

just an idea