The plain in which four tributaries join the Isker is the very heart of the Balkan peninsula, almost equidistant from the Adriatic, the Aegaean, and the Black Sea. Here the three great ranges of the West Balkan, the Central Balkan, and the Rhodope Mountains converge, and three important rivers find their source. The Struma flows south through Macedonia, the Isker north-east through a canyon of the Balkans into the Danube, and the Nisava north-west into the Morava. In the middle of the southern border of this plain, under the shadow of a lofty mountain, lies Sofia. The way to Sofia had been opened by the battle of Samakov. But its occupation was not the next logical step to Murad until the valleys of the Vardar and the Struma had been conquered. The occupation of Sofia was a temptation splendidly resisted in 1371. In 1381 it was a necessity. For it opened the path to trans-Balkan Bulgaria and to Serbia, and Murad was now ready to extend his conquest to the Danube by way of the Isker and the Morava. The Slavic chronicles are silent concerning the fall of Sofia. From the late Ottoman accounts, it would seem that the city was intermittently besieged for several years. Then a young Osmanli, who had entered the city as refugee, and had become the confidant and falconer of its commandant, betrayed him. He urged his master in a chase some distance in front of his followers, and fell upon him in a mountain gorge. The commandant was bound to his horse, and taken a prisoner to Ishtiman. Indje Balaban, son of the general of Osman who had besieged Brusa for ten years, brought his army from Philippopolis, and paraded the commandant, garrotted, under the walls of Sofia. The Bulgarians, discouraged and despairing of aid, surrendered.[379] We can be certain neither of the name of the Bulgarian commandant nor of the date of the surrender. But it was probably in 1385.[380] Bulgaria up to the main Balkan range was now Ottoman territory. The fall of Nish, in the summer of 1386, marked the next extension of Murad’s empire.[381] The Serbians did not yield without a struggle, as the Bulgarians had done. Nish was taken by assault. Lazar secured peace only by increasing the amount of his tribute and adding one thousand cavaliers to his contingent in the Ottoman army.[382]