From 1951 until the reunification of Germany, political prisoners were often detained by the East German secret police and subjected to conditions the Geneva Convention would not have approved of. The main prison center where this occurred was known as Hohenschoenhausen, an unremarkable complex of concrete walls and iron doors made remarkable for its collection of Cold War history and artifacts which are now displayed on public tours.
If you book a tour, you may walk through the interrogation rooms and the proliferation of prison cells and equipment to get an authentic look at how it was run under the GDR. There is also a guided tour, often led by inmates who have experienced it all and could testify to the conditions they faced, making for a truly genuine understanding of what Hohenschoenhausen really was. Tours are about $5 USD) and English tours only run once a day at 2:30pm on Wednesday, Saturday, and Sunday; so make sure you're there on time or you'll miss out on much of the grounds.