Its a common thing in games to be able to do this you can have units building as you lose them so when you get back from the battle replacements are already on there way but limits should be in place two that you cannot queue up more than your max army size. So players don't hide massive amounts of food in them.
This has been requesting many times in other threads please look through it before starting a new one as this just causes clutter.
This idea has been highly supported by the players.