You should include a small town Kotor in the list of places to visit in Montenegro. Kotor has fully retained its medieval appearance, with fortified walls and narrow streets (no more than 2-3 meters).
The land in the area of the Bay of Kotor (Boka Kotorska) in the mouth of the Skurda River was already settled under the Romans, but a town was founded there only in the period of the Byzantine Empire. The town was often destroyed and rebuilt, so Kotor acquired its today appearance only by 12-15 century.
If you want you can tour Kotor for two hours, but it is worth a much longer visit. In Kotor there remained medieval city walls, and not only around the city itself but also high in the mountains. In 15 century there were erected strong fortifications of the fortress of St. John on the rocks rising above Kotor, as at that time it was a border city and suffered from constant attacks.
There is no motor traffic within the Old Town. The streets of Kotor are very narrow and paved with highly polished stone slabs. All buildings of the Old Town are several centuries old. The main tourist attractions of the walled district of Kotor are the Church of St. Luke dating back to 12 century and the Catholic Church of St. Tryphon.
The first cathedral devoted to St. Tryphon was built in 1166 in the period of Byzantine Empire, but it was often rebuilt and destroyed (including earthquakes of 1667 and 1979). The cathedral has acquired its today appearance in 17 century.