In light of my garrison thread and the reading I've done, it seems to be completely coutner-intuitive to make cities have free upkeep slots and make castle units unavailable for free upkeep.
In the feudal system, knights had the duty to serve for military purposes up to 40 days without pay, and they also had a duty to garrison castles for a certain period without pay with a constant rotation so the castles were all well garrisoned. This suggests that it would make more sense to have free upkeep slots in castles to represent this duty that the knights always had.
On the other hand, cities would have been the exact opposite. In large cities, most notably in Italy, the wealth was highly concentrated and the population was large, so the cities actually paid for their own police forces. In other words, the "militia" units were actually paid to do their job rather than being akin to the shire levies that were called into duty during a time of crisis. This suggests that there really should be no free up keep slots in cities and the militia should be paid to do their duties.
Thoughts?
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