The Balkan Mountains have had a significant and special place in the history of Bulgaria since its founding in 681. It was a natural fortress of the Bulgarian Empire for centuries and formed an effective barrier to Moesia where most of the medieval capitals were located. The Balkan mountains were the site of numerous battles between the Bulgarian and Byzantine Empires including the Battle of the Rishki Pass (759), Battle of the Varbitsa Pass (811), and the Battle of Tryavna (1190). In the battle of the Varbitsa Pass, Khan Krum decisively defeated an enormous Byzantine army, killing Emperor Nikephoros I. For many centuries the Byzantines feared these mountains, and on several occasions Byzantine armies pulled back on approaching the Balkan Mountains.