Istria

Istria is in the west of modern Croatia. It was named after a tribe the Romans called the Histri. Protected by the difficult navigation of their rocky coasts they used piracy as a form of income and it took two military campaigns for the Romans to finally subdue them in 177 BC. After the fall of the Roman Empire, the Goths and Avars passed through, until the Lombards and then the Franks established themselves. 

In 1267 the coastal region was occupied by Venice, while inland was briefly Croatian, until the Habsburgs moved in. After the Napoleonic wars the whole region became part of the Habsburg Empire until that collapsed and the end of WW1. The post-war settlement gave Istria to Italy until 1947 when it became part of Yugoslavia. After the break up in settled into modern day Croatia.

While the Istrian coast is visited by thousands of tourists each year, particularly Porec and Rovinj, there are a number of castles inland that are worth a look.
​
In particular the medieval town of Pazin. And here are two more at Krsan and Sumber.

The main point of interest is the city of Pula (Pola Italian) in Croatia, which was the main naval base of the Austro-Hungarian Navy. There is plenty to see reflecting its varied history. Primarily Venetian influence until it became part of the Hapsburg Empire in 1813. Then back to Italy after WW1, before becoming part of Yugoslavia after WW2.

The Roman influence is most obvious in the well preserved amphitheatre.

The Venetian star fortress dominates the city and incorporates a small museum.

The best example of Austro-Hungarian fortifications is Fort Bourguignon. This is south of the city and tricky to find. Go through the reception barrier at Hotel Splendid and turn sharp right into their car park. You will see a pyramid structure on top of the hill and the fort is next to it. The guide book says that there are galleries with details of the fortifications around the city, but it was closed. I suspect the lack of signs means this is permanent.

Fort Bourguignon

Amphitheatre at Pula

Pazin

Krsan Castle

Sumber Castle

Venetian Fortress at Pula