The old city of Dubrovnik, which includes a large portion of the old walls, joined the Unesco list of World Heritage Sites nearly 40 years ago.
It would appear that the walls - built between the 12th and the 17th century - were never breached by a hostile army during the Middle Ages: the sea walls, whose purpose was to help defend the city from sea-based attacks (particularly from the Republic of Venice) are are 1.5 to 5 meters thick, while main wall on the landside is 4 metres to 6 metres thick and run an uninterrupted course of approximately 1,940 metres in length.
"Impressive" is a massive understatement.
This said, even if you're not into history or architecture, Dubrovnik is one of those destinations that should be on your bucket list, with its narrow, white alleys, its beautiful crystal clear waters, its excellent food...and the Game of Thrones filming locations (there, I've said it). Welcome to King's Landing, folks.