This book contains a definite divide between the country and the city, which both represent certain kinds of attitudes and mindsets. Cities (especially London) and even towns are used as a sort of code for certain types of behavior. For example, Fanny is "disposed to think the influence of London very much at war with all respectable attachments. She s[ees] the proof of it in Miss Crawford, as well as in her cousins" (45.12).
Fanny's use of the word "war" highlights the divisions and the conflict between cities and the country in Mansfield Park. Both the Crawfords and most of the Bertram siblings (except Edmund) are very "urban." They are also all witty, charming, scheming, flirtatious, not serious, and are fans of partying. This contrasts with the stricter views of morality that Edmund and Fanny share. Fanny's love of nature is also directly compared to Mary's "London" ideas and her greater interest in people over trees and shrubs (22.16-18).
We never actually visit London ourselves, though; we only hear about other characters going there through Fanny. This is pretty significant. The town and the city instead invade the countryside and often help reveal problems that exist in the seemingly idyllic countryside. Mansfield Park, in particular, stands in for the great country homes of the wealthy. Though it seems great on the outside, it has a lot of problems that the urban Crawfords help reveal. Instead of a noble estate, Mansfield is a place that's in financial trouble and that houses lots of family conflict. People there are often more concerned with money and personal gain than things like family bonds and moral duty.
The countryside definitely isn't immune to the influences of "urban" behavior, either. But the countryside also contains its own unique vices which often masquerade as, or pretend to be, virtues. Fanny and Edmund are the characters most explicitly linked with the countryside. And these characters have flaws that include close-mindedness and judgmental attitudes, things that those living on an isolated and wealthy country estate can develop.
The running theme of home "improvement" is also a very important part of the setting. We see and hear about various "improvement schemes" on a variety of other country houses (Sotherton, Thornton Lacey, Everingham, the Parsonage). The countryside, and particularly the wealthy people that live there, are in need of "improvements" themselves. (To read more about the "improvement" check out the "Symbolism, Imagery, Allegory" section.) Setting becomes a way to clue us in to major socio-economic issues of the time period, and it's also a way to help us better understand the book's major themes and characters.
The other major setting of the novel, Portsmouth, directly contrasts not only with Mansfield Park but also with the wealthy areas of London represented by people like the Crawfords. Portsmouth gives us a picture of working-class poverty and life on the coast of England, which is dominated by the navy. The noise, crowds, and poverty of Portsmouth stand in direct contrast to the refined and wealthy Mansfield Park. However, despite Fanny's inclinations, it's not easy to say if one home is really better than the other; both settings and houses have their own sets of problems and their own sets of positives. After all, the Price house may be chaotic, but it seems much more like a "family" home than Mansfield does at times. While Portsmouth contrasts with the other major settings of the book, it also helps to show how intertwined they are. And characters and themes overlap in all the major settings of the book.