He's a greater writer than any of us will ever dream of being, so I'll let Sir Terry describe Ankh-Morpork in his own words:
“Poets have tried to describe Ankh-Morpork. They have failed. Perhaps it's the sheer zestful vitality of the place, or maybe it's just that a city with a million inhabitants and no sewers is rather robust for poets, who prefer daffodils and no wonder. So let's just say that Ankh-Morpork is as full of life as an old cheese on a hot day, as loud as a curse in a cathedral, as bright as an oil slick, as colourful as a bruise and as full of activity, industry, bustle and sheer exuberant busyness as a dead dog on a termite mound.”
Here is a fantastic rendering of Ankh-Morpork if Google had driven their mapping cars around it (though I'm not sure how they'd have gotten out of The Shades with their wheels still attached) by a Tumblrer known enigmatically as Arse Garbage - fuckingfuckersfucked.tumblr.com
And I'm sure Kinja will shrink it to unviewable proportions so here's the full size version.