Based on the medieval written sources and the toponymy, we are able to identify the border zone between the Serbian and Bulgarian realms in the first half of the 14th century. According to the Serbian Archbishop Danilo II (ca. 1270-1337) the Bulgarian Tsar Michael III Šišman (reigned 1323-1330) invaded Serbian territory before the Battle of Velbužd on 28 July 1330, i.e. the area of the Upper Valley of the River Struma and the town of Zemen, which both belonged to the Serbian realm. In the Defter for the Sanjak Köstendil from the years 1570 to 1572 the region of Kraište is named "Gorno Kraište". Based on the toponymy, we are able to discern the respective area as a border zone between the two aforesaid realms. The renowned historian Konstantin Jireček (1854-1918) maps a toponym called "Kraište" to the North-West of Velbužd (today Kjustendil in Bulgaria), which as a term in itself denotes a border zone.