The Hohenschoenhausen, or STASI Prison, is well worth a visit. We were recently there with a school group of 16/17 year olds, mainly girls, and they reckoned it one of the best visits of the trip. Our guide was a young woman with excellent English, but as our pupils were German language students of mixed abilities, she did the tour half in English, half in German. (As previously mentioned, most of the guides are ex-inmates, but few speak English.) Allow at least 2 hours, if not more.
Fascinating, particularly the cells, including the water torture cell. Almost everything is still as it was at the time, and it doesn't need much imagination to visualise how it was.More