I shall define: Urbanism, n.: the art of putting ugliness into the heart of the citizens.
Every town alive of a certain size is ugly and dirty (excepting possibly Singapore, where they have death penalty for chewing bubble gum; but is that alive?). It has to be, and already follows from the entropy of all these masses sticking around at the same place.
They always were ugly, and full of dirt. Towns were the origins of every serious epidemic plague.
The real art, however, is making the people that live there accept it. Buildings, streets, sculptures: everything decays. The colorful and pure coating of a house will be ragged and dirty tomorrow. The glaring steel girder of a bridge will, decomposed by salt water, corrode into a dirty brown. The final rendering of a subway station will blister and lumps will fall of.
So why paint walls? Why polish steel? Why plaster the station?
And they stopped doing it, for it was a futile fight against entropy anyway.