I came across Ston by accident really. I was holidaying in the area and searched the WHS list and tentative list for any interesting sites nearby and so I "discovered" Ston. 
What is remarkable is that I have reached retirement age and never heard of the place before I did that search, I say remarkable, because the walls that surround Ston for 7 kms, are the second longest fortified walls in the world, after the Great Wall of China. The Croatians need to have a quiet word with their tourism marketing people.
The town itself is quite small but has some nice streets, a castle and an interesting church but the draw is definitely those 7 kms of walls that surround the town and were built to protect it's major asset, salt.
You can walk the walls if you want, it was far too hot for that while I was there so I confined myself to the town. You can see everything in the town in an hour but it is worth a visit if you are in the vicinity.