We've lived in Salisbury most of our lives and only visited Old Sarum on a couple of occasions.
The place is, of course, in ruins, maybe you could say "it'll look great when it's finished" . There was a time that it was finished, the best part of 1000 years so they say, so to have what remains is fascinating. The ground are well kept and the views are awesome over the surrounding countryside. Take your time strolling around soak up a bit of England.