We were planning to include Heidelberg on our itinerary, but many people who should know told us to swap Heidelberg for Rothenburg ob der Tauber.  We're glad we did.
Rothenburg's sad story is that during the Thirty Years' War (1609-1648) they had a little accident.  Whether it was an enemy cannonball or somebody being careless with matches, their powder magazine exploded tearing a great hole in the defensive wall.  Rothenburg fell to the invaders who took everything of value.  Left defenseless and poor, Rothenburg couldn't even repair the wall, so every invading army for the next 300 years occupied Rothenburg.
In the early 20th century the Burghers suddenly realized what they had: a place which had done essentially no modernization in 300 years.  They decided to keep it that way so that today Rothenburg is the perfect little medieval town, right down to the cobblestoned square.  Pay no attention to that Volkwagen van or the "No Parking" sign.