I think the only reason that the cities in the back are "unreachable" is because the city (or cities) on the frontline don't crumble very easily. If they did, those cities in the back would not be so protected, and might even be in more danger now if the frontline was holding the bulk of their military.
I think I'd prefer a game where a wide front and a narrow front were both viable options, but caused different types of headaches you have to deal with.
Ultimately, I think cities really just need to fall much much MUCH faster than they do. I mean, city flipping shouldn't happen because the game mechanics manipulate you into accidentally doing it. I think city flipping should happen because it's *really really* hard to stop it.
Maybe town centers should be attackable right away and flip the whole city when they are taken. Or even treat the whole town as something that is just taken over as a unit. Or maybe it can flip when a certain % of buildings are taken out. Having a city flip and instantly start producing units for the other guy is the type of thing that can snowball (theoretically).
If you felt your cities were constantly vulnerable, you would need to pump out military naturally in order to manage it. Yes, maybe you can build a deep resource-only city, but who knows where those bandits or woes are going to spawn, so it's a risky move.
I just think you can have something more "organic" than a number you have to manage or arbitrary rules forcing you to play in a specific way.