 The country 's military and economic might declined after the Asen dynasty ended in 1257 , facing internal conflicts , constant Byzantine and Hungarian attacks and Mongol domination . By the end of the 14th century , factional divisions between the feudal landlords and the spread of Bogomilism had caused the Second Bulgarian Empire to split into three tsardoms — Vidin , Tarnovo and Karvuna — and several semi @-@ independent principalities that fought each other , along with Byzantines , Hungarians , Serbs , Venetians and Genoese . By the late 14th century the Ottoman Turks had started their conquest of Bulgaria and had taken most towns and fortresses south of the Balkan mountains . 
