The Studentenkarzer — the university's old student lock-up — is housed in the Auditorium building near the Wilhelmsplatz. From 1812 to 1933 the university used these small rooms to discipline misbehaving students, and the inmates left their mark in the form of painted graffiti: regimental crests, fraternity colours, satirical drawings, and signatures of generations of doctors and lawyers-to-be.
Access is by guided tour from the university's main building, usually weekly in summer. Worth timing a visit around.