1. Ankh-Morpork begins as a crossing point at the river Ankh, on an established route between towns Hubward (and later Sto Lat) and points Rimward. The route is still visible on what are now Broadway and Edgeway Road. The tall Tower of Art, from an earlier time, made the crossing point easy to find. A ford, usable at certain times of year when the river level was low, is eventually replaced by a year-round cable-ferry. Such a crossing point would become a confluence of local and long range routes, being the last crossing point of the river before the sea, (similar to Tower Bridge in London) and would attract heavy traffic between towns and villages along the coast when the sea is too rough or the wind is in the wrong direction for leaving harbour.
2. The ferryman’s hut is quickly joined by a black smith and an inn, damaged carts and weary draught animals are exchanged and traded. The crossing of many paths would make this a natural trading point, where goods from far-away places as well as local produce would be bought and sold, cargoes would be split and recombined. In time the growing town would also become a destination in itself as ships ran up the river to unload goods that would be distributed to settlements inland by craft from up-river. A toll bridge over the river would be an paying asset worth protecting and the river would provide power for mills processing grain and timber.
3. The peninsula of the Isle of Gods with the river on 3 sides made a easily defensible area, especially once “The Cut” was made, isolating the early city in a fully moated position.
Below; possible pattern of growth from origins to present size. City walls are built/moved to encompass new areas of the expanding city as those areas achieve the size and density and hence the political power to make demands, if the city rulers want to collect any taxes.
Built on loam as the city is, one would expect the same expensive and scarce stone to be re-used in the new city walls. The city sheds and recycles it’s skin like the hideously malformed but vigorous result of a late night accident in the laboratory.