The city is something like an ecosystem, with its webwork. This, in turn, could be viewed as a metaphor for hypertext. This site is as hypertext, is as a metaphor for the city.
People within the city interact on numerous levels -- a hierarchy of city life -- just as my web works on various different levels. Within the webwork, there are roads and highways, streets and networks of subway systems. Trains run through the web, and wherever you are, 
you can always pick a different way to go.