East Berlin built this boulevard to be read like a poster, and most people walk it without reading a single line. Pick a stop on the U5, then tap the numbered pins on the real facades. I have marked the details I point out when friends visit.
Take the U5 to Strausberger Platz and walk east to Frankfurter Tor. That is the 1950s showpiece stretch, about 20 minutes without stops and 45 with them. Kino International and Café Moskau sit on the newer western half near U Schillingstraße, so see them first or last with a one-stop U5 hop. Full boulevard, Alexanderplatz to Frankfurter Tor, is 2.3 km. Read the full guide at berlinwalk.com/post/karl-marx-allee-berlin.