Berlin is dotted with memorials and reminders of its troubled 20th-century history. For a man with such megalomaniac ambitions, it’s striking how little survives of the world Hitler created. This spot, once the most feared address in Berlin, documents the methods and evils of the Nazi regime. In early 1945, as Allied armies advanced on Berlin and Nazi Germany lay in ruins, Hitler and his inner circle retreated here. The statue of a pietà, Mother with her Dead Son, is by Käthe Kollwitz, a Berlin artist who lived through both World Wars.