Base builder, persistent levels

Hi everyone,

I've been puzzling over the best way to implement this and was wondering if you could offer some advice.

Essentially I want players to be able to build up their own base as part of a much much larger world, players can interact between themselves by sending troops/resources etc.

What i'm struggling with is how I deal with offline combat/troop movements, lets say when i'm online I send a load of troops to sit by an enemy base and pick off incoming outgoing troops(like harvesters etc) I then log off so neither me or my target is online, obviously the harvesters are still going to need to move, combat is still going to need to happen?

The only way I can think of is have some kind of master client that takes over the units when nobody is online to control them, but the world is split into a scene for each base then scenes for the terrain in between so how could I connect a master client to every base?

The reason I decided to have a scene for each base is it seemed easier because when the player connects he simply downloads the position of all of his buildings etc from our servers also i'd like to think we'd get a few thousand players so having a huge world that will be big enough to hold all their bases just isn't going to be practical especially since we want to allow mobile players to join in too.

Any suggestions would be greatly appreciated i'm not adverse to a rethink on how bases work and are represented since having everyone on one level would make life a hell of a lot easier but thats going to cause problems for mobile clients i'd assume?

Thanks for any suggestions in advance

Marc

EDIT

As an idea and i'd like some input if it would be the best way, i'm using Photon for networking:

You have a Master client connected to each of the "World" zones so that can deal with moving autonomous vehicles and troops within the world where the player isn't connected to that level(ie if the player is messing around with his/her base and has troops in another zone which he's ordered the master client moves them until he reconnects)

To keep things simple you prevent base assaults where no players are connected.