This seems like it could become frightfully complex. I like the idea of making cities a seperate chunk of land, but having a lot of siege-specific commands seems like overkill. I want to only worry about modeling strategic combat, and not have to make tactical or operational decisions. Since a turn represents a year, and most sieges were resolved in a year, there isn't any need to get this detailed. I would rather just give an order to attack and let the math run in the background.