What makes Berlin stand out to me in comparison to every where else in Europe is the fact that the city is truly the nexus of two major events in 20th century European history - the Second World War and the subsequent rise of the Iron Curtain in the East.
Nowadays, Berliners seemingly make little effort to mask either dark chapter. One tour that should be considered essential to any visitor of the city is a trip to the Sachsenhausen concentration camp just to the north of Berlin. What made the biggest impression upon me when I visited was just how easy it would be for Germans to simply raze this awful site to the ground and pretend that the atrocities that took place here never happened. But instead, there is a sense here that it is more honorable to wear one's history on their sleeve, regardless of how reprehensible the past might be. There is an important lesson to be learned through understanding that a place like Sachsenhausen exists, because people who do visit these places gain an understanding that the events of the Second World War and the Holocaust must never happen again.
The words "Arbeit Macht Frei" (loosely translated to Work will make you free) would greet prisoners brought to Sachsenhausen during its time of operation from 1936-45.
Another more out of the way, but no less chilling of Germany's darkest chapter in history can be seen at the Grunewald train station. On the way back from the Gribnitzee campus, the S7 train makes a stop at Grunewald where visitors can see a quiet memorial to where thousands of Jews were transported by train off to extermination camps in Poland during World War II. Although rarely indicated in any Berlin guidebooks, bronze plates along the tracks provide a reference to the dates and the number of Jews transported out of Berlin.
As compelling as the impact of World War II is on Berlin, one cannot forget the division of the city under communism after the war.
Standing these days in Alexanderplatz is a display commemorating the events that led to the fall of the Berlin Wall in the Fall of 1989. To me, these events have such relevance because they actually happened in my lifetime. I'm old enough to remember there actually being a country called East Germany, and taking the time to walk through this display chronicling how stressed East German society became gave me a better understanding of events that I saw play out on the news when I was very young.
Photograph at the free display in Alexanderplatz shows an East German couple escaping to Hungary mere months before the Berlin Wall fell in November of 1989.
Visitors to Berlin should not miss the DDR Museum! This is an interactive museum designed to show what life was like under communist rule. Here, one can sit in an East German made Trabant (basically the East German response to the West's Volkswagon Beetle), watching Stasi (East German secret police) surveillance video, and even sample a variety of programming from the DDR era. The fact that you can actually pick up and hold much of what is on display at the DDR Museum makes it a fun experience, and provides fantastic insight into history that is not that distant.