Concentration camp built by the Nazis in 1936 in Oranienburg, town located less than 50km away from Berlin. It is quite simple to get there: you just need to take the #1 S-Bahn line (S1) all the way to Oranienburg Bhf (which it is the last stop, easy). Once there, the memorial is only a 20-min walk away but it is also possible to take the bus #804 direction Malz and get off at "Gedenkstätte" bus stop.
Photo: view over the prisoners' main field and what was one of those 68 barracks.
I must say this is one of the most impressive places I have been so far to. It looks so huge and empty nowadays that it seems almost impossible to imagine how full it used to be. For us to have an idea, the memorial has replaced the total amount of 68 prisoners' barracks by filling the spots with grey stones (photo above). There are 4 rows of them in the main camp plus 18 other located in the "small camp" - a part of the concentration camp that was built due to the increase of prisoners.
If I remember well, there are still two barracks standing in the mentioned "small camp" and, even it is impossible to imagine the conditions in which the prisoners were treated, you can still see bunches of 3-floor bunk beds inside those tiny rooms, leaving any space to move.
The first ones to get there were mostly political prisoners against the Nazi regime, but soon would also start arriving people considered racially or biologically inferior to "Aryan race" such as Polish and Soviets.
The entrance to the area is free but, if you like history, you should not miss their guided visits (14€) in English, German or Spanish.