Formerly considered East Berlin, Karl Marx Allee is one of the best places to get a glimpse of socialist history, architecture, and local life in Berlin from the past. The famous street is 89 meters wide and 2 km long and still houses some of the "wedding cake" architecture that East Germany was known for.
You don't need a guide. Save your money and simply check out the numerous (30+) plaques along the way (on both sides of the street) which provide you with history, stories, and old photos of the famous street. I suggest that you start at the Alexanderplatz U-Bahn station and walk along the north side of the street, then return along the south side—to get the full scope of the ultra-wide street.
This Soviet-built boulevard starts at Alexanderplatz and extends some two kilometres far out into the borough of Friedrichshain. Formerly known as Stalin Allee, the expansive street was seen as East Berlin’s answer to the West’s famous shopping street, the Ku’damm. Today it’s a protected landmark whose imposing communist architecture still houses residents as well as new venues like art galleries, restaurants and the wonderful computer game museum.