Complex sentence: Peder Hansen Resen, who was appointed lord mayor of Copenhagen in 1664, wrote several works about the history of Denmark as well as Copenhagen and he is the principal source of knowledge about the use of the city hall.
Split into simpler sentences: Peder Hansen Resen, who was appointed lord mayor of Copenhagen in 1664, wrote several works about the history of Denmark and of Copenhagen. He is the principal source of knowledge about the use of the city hall.