Here’s a great bit of writing from Engels to start us off:

Such is the Old Town of Manchester, and on re-reading my description, I am forced to admit that instead of being exaggerated, it is far from black enough to convey a true impression of the filth, ruin, and uninhabitableness, the defiance of all considerations of cleanliness, ventilation, and health which characterise the construction of this single district, containing at least twenty to thirty thousand inhabitants. And such a district exists in the heart of the second city of England, the first manufacturing city of the world. If any one wishes to see in how little space a human being can move, how little air — and such air! — he can breathe, how little of civilisation he may share and yet live, it is only necessary to travel hither.

From The Great Towns

I think Engels might note that many of today’s vociferous advocates of free market cities have never actually lived in free market cities, and that if they had, they might have different opinions.