Castle Starkenburg

 The castle ruins at Starkenburg (Heppenheim), I would put on the 'if you were passing through' list.  

First, the location is Heppenheim (20 min south of Darmstadt).  Once you enter the city, it's about a 15-minute drive UP the mountain side on a marginally 'nice' road (mostly brick), at a very low rate of speed. Don't make the trip in the winter.

Second, once you reach the top of the hill, there is the parking lot.  From there, figure about 20 minute walk.  If you have knee or hip issues....cross the castle ruins off your list.

Third, there is a decent restaurant and pub at the top, and I'd highly advise you to sit for a while....chill out, and at least consume a beer.  

Fourth, the ruins?  They don't allow you into the interior of the castle....lot of renovation work underway (summer 2020 period).  You can walk the grounds and get an idea of the character of the castle.  Beyond that....nothing much else.

If you live within an hour of Heppenheim, it's definitely a good 3-to-4 hour drive/walk.  Beyond that, no....I wouldn't put it on the must-do list.