Duklja Ruins is an unusual attraction for Montenegro. It's the ruins of the ancient Roman city. Nowadays, it's located near the capital
Podgorica and the entrance is free.
The city had a beneficial location - surrounded by mountains, it was protected from enemies. It was an important trading center of the Roman Empire. It's not much left to nowadays - you can see remains of columns, forum, basilica, buildings, walls, temples etc. Most of the city fell underground. What is left is still pretty impressive - the skills that were used to built so precisely perfect shapes of columns, walls etc admire. The feels of the real Empire is everywhere. It's highly recommended to visit.