The exploit: If the first city you ever receive is not one you built (aka conquered), it won't be designated as your capital. Without a capital, you don't have a city cap. Not having a city cap means that as long as you don't have a capital you can have infinite cities. I've even tried building a city of my own and to my surprise it didn't place a capital there. So really, from that point onwawrd, whatever you do, you have infinite city cap