Complex sentence: Novo Brdo was one of the largest cities in the Balkans, and because of its rich gold and silver mines it was the most important non-coastal city on the Balkans in 14th and 15th century.
Split into simpler sentences: Novo Brdo was one of the largest cities in the Balkans. Because of its rich gold and silver mines Novo Brdo was the most important non-coastal city on the Balkans in 14th and 15th century.