Removing population criteria is important so you don't do organized invites long enough to survey, then move on.

I think that having an input resource, to get an output resource
ex. to get a coal resource, you need to have a food, iron, and cloth resource to feed and supply the 'miners'.

To get a pork resource, you would need pig food.

What you would get resource wise is much more then what you put in, making it a worthwhile expense.

This would make trade more important, and I think it would add some activity on non-siege days, as the town resource is more involved then the mayor typing in a command.