I think Dev's will need to make cities SAFE zones or NoN-Combat zones.
I would gladly help build a city but if griefers can come destroy it and my work then I will not waste my time.
Its different when you build your base, you expect to be attacked.
With a city that would benefit all trade hubs etc. then you would expect a safe zone to fly over that city/planet
I think thats the only way to go .