Earthworks built in the seventh century on the border between the Bulgarian Empire and Bizancium. "Erkesiata" is the largest and most famous protected fortification south of the Danube, in the area of the vast territory of Bulgaria during the VIII - X century. It is part of the system of fortified shafts, built in all border buffer zone of the Bulgarian state.
http://bulgariancastles.com/bg/node/1125
http://www.yambiz.com/ibm/index.php/en/tourist-destinations/172-erkesiyata