After the death of Stefan Dušan on 20 December 1355, the Serbian Empire lost much of its hegemony in the Western Balkans and the large and ethnically diverse empire split into several successor states. The Bulgarian and Byzantine Empires of the period were once again the only remaining major political powers on the peninsula with the potential to stop the Ottoman expansion. Between 1354 and 1364 the Turks conquered Thrace as a number of important fortresses and towns, such as Plovdiv and Stara Zagora fell under attack. From the end of the 1350s Ottoman military units even reached the surroundings of the capital as, according to sources, the Emperor took precautions to strengthen the city walls. Ottoman chronicler Seadeddin suggests that Turkish advance between 1359 and 1364 involved destruction and depopulation of many areas: the towns Plovdiv, Stara Zagora, and Sliven were devastated, and others such as Venets and Sotirgrad were completely destroyed. Destruction was accompanied by slaughter and deportation of the local populace to Asia Minor.