King Tvrtko I of Bosnia, house of Kotromanic. Under his rule, Kingdom of Bosnia was formed and expanded on territories of modern day Croatia and Serbia, same as Montenegro. Bosnia was the most powerful kingdom in the Balkans, with stable economy, powerful army and navy. It also managed to inflict first heavy defeat on the invading Ottoman forces in the Battle of Bileca, 1388. When 7000 Bosnian knights broke the army of 20 000 Ottoman soldiers led by Lala Sahin Pasha. Good old days.