King of Prussia
The Pennsylvania Turnpike (I-76) takes you right to it on the way to Philadelphia.
While it may seem like the only way to see anything is to drive your own car, this turns out not to be the case. This traveler was pleasantly surprised by the SEPTA (SouthEastern Pennsylvania Transport Authority) buses and trains. Bus 125 runs from KoP directly into the center of Philadelphia. You can catch it along Gulph Rd. You will arrive near the train station at 30th St and Market St; from there, you walk a few feet to the station itself and get on the eastbound subway towards the Old City and other attractions. As of 2008, the bus fare is $2 per trip. The buses run every 20 minutes during peak hours and every hour off-peak, during the weekdays, and the trip takes around 40 minutes.
While taking a bus may seem inconvenient, you will save time and hassle by not driving into Philadelphia yourself. Relax and let the bus driver fight the traffic. You will save even more time and money since you don't have to find a place to park your car in the city!
In the reverse direction, the last bus leaves Philly (again at 30th and Market) at about 10 pm, on weekdays. Check the SEPTA website  for specific information.
There is a train station in neighboring Conshohocken, at Washington and Harry. Regional Rail R6 will take you to the city center. The train ride takes about 30 minutes, and the fare is $5 if you buy tickets on board ($4 if you buy in advance). Check SEPTA's website for timetables.
The station has a park-and-ride lot, but spaces may not be available to tourists.
There is also a train station in KoP, on Gulph Rd southeast of the mall. However, it would be a long and dangerous walk from the mall area, and there may or may not be short-term parking there.
Traffic in this area is thick and fast-moving, especially during rush hours (7-9 am and 4-6 pm). Expect the interchanges to be backed up to the point where there are lines of cars winding around the nearby neighborhoods. Try to familiarize yourself with the traffic flow before you try driving in it, because if you are in the wrong lane trying to get onto one of the freeways, you will probably not be able to move over and will have to go the wrong direction and take a detour. Gulph Rd and Schuylkill River Rd are winding, steep, and have suprisingly high speed limits even though there are houses on them. Watch out for hidden driveways and sudden stops.
Be careful walking around the town. There are few crosswalks or pedestrian signals. Near the mall, you will be forced to jaywalk if you want to get anywhere on foot. It is not a pedestrian-friendly area by any means.