Complex sentence: To this Wagner added material from Hoffmann's story, from "Serapions-Brüder" (1819), describing a song contest at the Wartburg castle, a castle which featured prominently not only Thuringian history, but also that of Germany as a whole.
Split into simpler sentences: To this Wagner added material from Hoffmann's story, from "Serapions-Brüder" (1819), describing a song contest at the Wartburg castle. The Wartburg was important in not only Thuringian history, but also that of Germany as a whole.