There are many telling signs that distinguish European cities from American cities. The main sign is the age of the architecture. American cities are a mere 250 years old (at least the oldest ones), while their European counterparts are measured in many centuries. The other sign to look for are the churches. It is not hard to find the churches in European cities, as they are usually near the center of town and are typically the largest building in the city, sometimes by a sizable degree.
The Catholic Church (Church of the Holy Spirit) in Heidelberg is a prime example. Looking at the city from above, the church dwarfs the other buildings in the city by a wide margin. Building on the church took almost 150 years until it opened in 1544. Unfortunately, we didn't have time to step inside to view the architecture. I was told that you can climb to a viewing platform in the tower that gives the best panoramic view of the city. Maybe on the next trip.