Battle of Iatrus

The ancient and medieval fortress of Iatrus stands near Krivina village on the eastern bank of the Yantra River, on a strategically important elevation. It is mentioned in Roman road maps as early as the third century. The fortress is shaped like an irregular heptagon, covers about 30–40 decares and is reinforced with twelve towers and a massive defensive wall. Its main entrance is on the east.

In 535, the first large-scale Slavic invasion of the Balkan Peninsula began. Numerous Slavic detachments attacked and ravaged Thrace. Byzantium managed to repel the Slavs, but the Slavic presence in the Balkans left lasting consequences. Meanwhile, a Bulgar army attacked the lands south of the Danube. A clash occurred between the Bulgar detachment and the Slavic forces.
 

The Bulgars were routed. Their defeat was due both to the Slavs' numerical superiority and familiarity with the terrain and to the possible inadequate preparation of the Bulgar forces for such resistance. This defeat temporarily frustrated Bulgar ambitions to penetrate farther into the Balkans and establish themselves there, although in later decades they would return as a decisive factor in the region's historical development.