How can I architect a persistent RTS using Unity as the client?

I'm looking to start working on an RTS using Unity, though the idea I am working on requires that the RTS to is persistent. So when the user exists the game (client), their units continue to exist and participate in the world via the unit AI. The user can then log back in and resume control of units.

I realize that there will need to be a game server, and the user clients will connect to that server, but my experience with Unity is limited. I'm an experienced software developer, but not in game design.

I'm hoping people are able to suggest how to architect such a game. What would the server look like? and how would it run the world? What would the client application look like and how would it talk to the server? and where / how does Unity itself fit into this scheme?