I visited the prison while I was still a student and I found out that it wasn't that bad and that actually students wanted to go to prison so that their art would forever be on the walls of the prison and of course, to skip classes.
Besides the fact that it was dark and unwelcoming, it was an interesting experience. The "Studentenkarzer" can be visited daily and the ticket costs 2.50 euro, reduced for students :). Also, in Romanian we have "carcera" which is pretty similar to the German, "karzer".
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