[E]very city begins as a slum. First it’s a seasonal camp, with the usual free-wheeling make-shift expediency. Creature comforts are scarce, squalor the norm. Hunters, scouts, traders, pioneers find a good place to stay for the night, or two, and then if their camp is a desirable spot it grows into an untidy village, or uncomfortable fort, or dismal official outpost, with permanent buildings surrounded by temporary huts. If the location of the village favors growth, concentric rings of squatters aggregate around the core until the village swells to a town. When a town prospers it acquires a center — civic or religious — and the edges of the city continue to expand in unplanned, ungovernable messiness. It doesn’t matter in what century or in which country, the teaming guts of a city will shock and disturb the established residents. The eternal disdain for newcomers is as old as the first city. Romans complained of the tenements, shacks and huts at the edges of their town that “were putrid, sodden and sagging.” Every so often Roman soldiers would raze a settlement of squatters, only to find it rebuilt or moved within weeks.
The Choice of Cities, Kevin Kelly