While citizens cannot be transported, Spearmen can, and relatively inexpensively. Dismissing a Spearman adds to the population in the town where he is dismissed. You can take advantage of this to achieve various objectives:
You can storecitizens temporarily, i.e. to avoid losing them if you run out of .
If preparing for a large, long-term war, you might start out by recruiting as many spearmen as possible, then begin recruiting your main forces. This drastically increases the satisfaction level, which helps your population grow faster. After you've recruited most of your main forces, you can dismiss your Spearmen and your population should be near their maximum again.
Just remember that Spearmen can be killed; citizens can not.