Franklin begins his story in Boston, MA and ends it in London, England, but spends most of his story in Philadelphia, PA. In the Autobiography, Philadelphia's development into a "grown-up" city parallels Franklin's transformation into a diplomat and businessman. Just as Franklin identifies weaknesses and flaws in his education and business plans, and sets out to fix them, he sees defects and lacks in his young city and does the same for it.
For most of the Autobiography, Franklin's teaching Philadelphia how to be a proper city. Because Philadelphia's so new and raw, it's the perfect place for Franklin to work on his diplomacy, civilization building, and planning skills, and he flourishes in it. He wouldn't have nearly as much to do in Boston, under his brother's thumb, where he'd always be the junior partner. The same goes for London, which makes room for Franklin up to a point, but already has its own systems, organizations, militia, and universities. Philadelphia's a blank slate where Franklin can make his mark. In fact, Franklin fashions just about every public service agency and office in the city, most of which are still successful in the twenty-first century. Franklin has such a hand in shaping how the city works and functions that visiting it is almost like being in his home.
Philadelphia can also be seen as a kind of Celestial City, like in Bunyan's Pilgrim's Progress. For more on this, head over to our section on "Symbols, Imagery, Allegory."