I like what I've read thus far. I think that garrison units that spawn under siege should be immobile, crappy militias who have fled the country to defend the city. But I think it should be up to the player to make sure that better trained units are nearby to meet any invasion.
These spawning of these units should take away 1000 minai to represent the arming and feeding of these refugees turned soldiers.

A script to disband them after the siege would be awesome, as they would return to being productive citizens.

Keep coming up with these awesome ideas!