I visited this transnational WHS mainly in April 2014 (Croatia and Montenegro), in August 2013 (Bergamo, Italy) and countless times but specifically after inscription in July 2017 (Peschiera del Garda).
I still cannot understand why Hvar, Croatia and mostly the Venetian fortifications near Venice itself have not been inscribed (perhaps they will at a later stage with an extension).
The Fort of St. Nikola is on the outskirts of Sibenik and is worth visiting if you're in the area. The fortifications of Kotor are by far the most impressive both at street level but especially if you climb to the top. The Castle Of San Giovanni offers incredible views not only of the fortified city of Kotor (photo) but also of the inscribed Bay of Kotor.
In Italy, I visited Peschiera del Garda practically every other summer weekend since I was a child but I revisited specifically after inscription in July 2017. It is a pleasant fortified town by the Lago di Garda but I would never have imagined that it would make it to the WH list. The same applies to Bergamo which I visited en route to Crespi d'Adda.
I wouldn't mind visiting Zadar and Palmanova in the future to complete all inscribed locations but it will have to be a combined visit with another WHS so it won't be any time soon I'm afraid.