The Hohenschoenhausen Prison was a recommendation from Trip Advisor. Not something in the guide books, but very worth it and interesting. You should look it up and read a bit about it. In short it was a prison used by the Soviets and then the GDR onward to detain political activists and other "unwanted's" right up until 1989.
In the picture to the left of Ashley is David and Mary. A lovely couple we met in Berlin through the local church. They came along with us. David worked for the US military in Wester Berlin during the cold war. It was really interesting learning some first hand stories from him.
One catch though. The guards caught on. Guards would play the part of fellow inmates and communicate through the walls. They would then learn the inmates fears and desires. Once known they would be used against them during interrogation sessions.