The Heiliggeistkirche church is only one of many large and small churches, but definitely the one with the most interesting history. During the Dark Ages, it was the shelter of the Bibliotheca Palatina, Germany's oldest library. The Bibliotheca was stolen and brought to Rome but eventually returned in pieces. Today, parts of it can be visited in the University Library (also the oldest and probably the most valuable of its kind in Germany), which is situated close to the old university. You can get a great view of the Heiliggeistkirche, Old Town, and the Neckar river bridge from the castle (Schloss Heidelberg).