All ambitious missionary movements are, or soon become, urban. If the goal is to "make disciples of all nations," missionaries need to go where there are many potential converts, which is precisely what Paul did. His missionary journeys took him to major cities such as Antioch, Corinth, and Athens, with only occasional visits to smaller communities such as Iconium and Laodicea. No mention is made of him preaching in the countryside.- Rodney Stark, Cities of God
Lots more meaty info here about the spread of early Christianity among an urban context. For instance - population density of Antioch around 117/acre, compared to Chicago 21; SF 23; NY 37; Manhattan 100.