Uhm...I never said anything about multiple cities
If you mean the plural with level 20/40, I was just referring to the level of the player's city. I'm not saying he controlled
In any case, it wouldn't be that costly-after all, players would earn bonuses for controlling such cities. You could do all kinds of creative things with a feature like this. It'd just need a little smoothing over to get rid of the edges.
Though you're right, the players army would get expended on taking one of these and in competetive play it would theoretically leave the player open for counter-attack after taking the city. A curious and difficult thing to work around, but chances are the fact that the enemies whole army + the player's whole army wouldn't be fighting- it'd just be a fraction of it. What sane person would leave their main city completely open in favor of a border town?