A big square right in the center of Berlin - in the Golden 1920s it was at the heart of the city's nightlife (and inspired the Döblin novel of the same name, which in turn is the basis for Fassbinder's TV series).
For several decades it then belonged exclusively to East Berlin (and may have become a little drab during that time).
Since 1990 it's again right in the middle (and in the district Mitte) of the reunited city. It may be missing the exciting nightlife - but it still offers a lot of shopping opportunities (especially in near-by Alexa shopping mall) and great people watching. It's also a big hub for the trains and trams and, last but not least, is overlooked by one of Berlin's most famous sights: the Fernsehturm.