I like your system SG. I'm assuming seigers can't be attacked by players inside the seiged city and conversely those trapped in the city can't attack those doing the seiging until the wall, or any city defenses, are down.
Just to add to it...
- As long as there is a hex unoccupied around the city being seiged, any player (friendly player that is) may leave or enter the seiged city through that hex.
- If enemy players hold a city, a resistance of NPCs would be available to aid those of the city's element to take back the lost city.
I'll post more on this in another thread meant for this kind of thing. Don't want to derail the thread too much