As the the game is now, roaming bandits and monsters are not very aggressive toward cities. Sure they are a threat and players feel the need to leave a garrison in them, which is a BRILLIANT concept that I have really never seen in any other empire building game, where players don't really need garrisons thus ending up being perfectly fine even without defending their empire.
By being forced to create an infrastructure to defend their newly created outposts players would have to dedicate time, armies and resources every time they build a new outpost. That could be a deterrent to spam cities without having to create unrealistic rules.
Another way could could be that in order to create each new outpost the player would have to research it's "new city planning" as a new tech, thus having to spend time researching each time a new outpost is going to be created.
Again, in order to be realistic, as I suggested in another post, towns should contribute more with their taxes to the economy of the empire with TAXES, but they would also have an upkeep. By having coaches that carry taxes to the capital (in a way similar to that created for merchants).
Player would have to defend those coaches, by defending the routes and/or adding an escort to each coach. The loss of a coach would thus become an economic loss for the empire and outposts that are too far to be defended would soon become a weight for the empire. (a transport by boat should be created for those town that are not reachable by land). Maybe by researching a new building the "tax collector" in level 4 settlements, players would be able to choose which town the taxes go to, other than the default starting city.
Any other idea?