Complex sentence: Among the treasures that are not shown to the public is a collection of photographs, which was built up during the 1920s by Carl Georg Heise; included are approx.450 artistic photos, among them are 212 photos by Albert Renger-Patzsch.
Split into simpler sentences: Among the treasures that are not shown to the public is a collection of photographs. The collection was built up during the 1920s by Carl Georg Heise; included are approx.450 artistic photos, among them are 212 photos by Albert Renger-Patzsch.