The eastern part of the city was built when Philly was the US capital, in the first years of the XIX century. Much of this area has been painstakingly restored, and is incredible to walk. Many streets have cobblestones, and most are lined by beautiful streets. This area is also very clean and safe (just beware of many crazy drivers).
Fondest memory: This city offers the benefits of a large metropolis, but does not overwhelm like NYC.
As you walk NW towards the City Hall, the Federal style townhouses are mixed with the newer Belle Epoque architecture, providing for a nice transition.
Fondest memory: This city is actually excellent to study architecture through history.