So here's the deal... If you have run out of things to do your first day, or you're going to be in town for more than one day, then this is a cool place to visit.
In the old-days, when ONLY the sons of wealthy & powerful men were able to attend a university, this "jail", is where they would be sent to serve time for petty legal transgressions, misdemeanors, if you will. But because their fathers were rich & powerful, they of course got special treatment & came here, instead of peon-prison.
Add this to the fraternities' practice of intentional facial scarring, and you've got yourself some pretty interesting stuff.
They have the best Universität Heidelberg souvenir polo shirts. So if your child is a student in Heidelberg, come here for your Uni-wear.
Come here if you have already toured the castle, including the Apothekenmuseum, been up the Bergbahn (funicular) to Königstuhl, seen the Heiliggeistkirche (Holy-Ghost church), walked the alte Brücke (old bridge), gone to the other side of rhe river to see Thingstätte (Roman ruin near outdoor amphitheater) & if your able, walk the Philosophenweg.