Walter then returned in 1900 to Berlin , where he assumed the post of Royal Prussian Conductor at the Staatsoper Unter den Linden , succeeding Franz Schalk ; his colleagues there included Richard Strauss and Karl Muck .
He then went to Riga , returning to Berlin in 1900 where he conducted at the Staatsoper Unter den Linden where he worked with famous people such as Richard Strauss and Karl Muck .