Over at the blog Ex Urbe, there’s a long, chewy post (with pictures) about the historical development of the city of Rome from its first days as a cluster of huts on a hilltop by the Tiber.
Writers dealing with invented worlds (whether past or future), take note: This is how a real city grows up. Your invented cities need to have similar layers to them if you want them to feel real. (This is also, I suspect, why planned cities can have such a flattened feel to them. They haven’t had enough time in place yet to accumulate additional strata, so when you scratch the surface all you get is more surface.)