Dubrovnik, Croatia - Dubrovnik is a city on the southern Croatian coast of the Adriatic Sea. It is the world’s greatest fortified city. The old city is fully enclosed within massive stone walls which stretch for 1,940 meters. The walls are up to 25 meters high, and in places even 6 metres thick, especially in those places most important to the defence of the city. There are five fortresses in the old city, three of which are part of the surrounding wall.