Callisto presents a simple (almost too simple) straightforward approach to running an email campaign in any setting. Military, financial and popular support issues are handled very simply, and may not contain enough detail to satisfy those who want to know how many arrows each of his archers is carrying, but it works for me.
The one peculiarity of the system is that it leaves a lot of the world-running in the hands of the players. For example, they recommend that one player be in charge of determining who won any battles that take place in the campaign. They suggest that no player be put in charge of an aspect of the game that can affect their character, but unless some player is running Switzerland, I was unclear as to how that would work.
Generally, however, it is a decent system in the modern "diminished GM role" tradition. I recommend it, if only to get some additional ideas for your own email campaign.