The Old-New Synagogue in the Jewish Quarter is the oldest functioning synagogue in Europe, and was built around 1270. It is famous for the story of the Golem of Prague.
Supposedly, the Rabbi (Judah Loew ben Bezalel) created a golem out of mud. When a Nazi officer went in to the attic (or Genizah, a storage place of old writings), the legend says the Golem came alive and killed him. Supposedly during the war the Gestapo didn’t enter the attic and the church was preserved. The attic isn’t open to the public, further preserving the mystery!