In 1380, Murad ordered the advance to the Vardar. Istip was captured, and colonized in the same thorough way as had been done at Drama and Serres. A large army under Timurtash crossed the Vardar, took Monastir by assault through the marshes, and pushed north to Prilep.[372] Monastir and Prilep became frontier fortresses of the empire. The conquest of Macedonia was now complete. These cities were excellent bases of operation against the Albanians to the west and the Epirotes to the south-west. During the reign of Murad, the Osmanlis did not attempt a subjugation of Albania and Epirus. They were, however, invited into these countries by native princes. Thomas, despot of Janina, used Ottoman mercenaries against the Souliotes in 1382.[373] Two years later, after the assassination of Thomas, the Albanians besieged Janina with Ottoman aid.[374] The civil war that arose around the widow of Thomas prepared the way for the Osmanlis to extend their rule to the Gulf of Arta. In 1385, Khaïreddin pasha, who had occupied Okrida, the ancient ecclesiastical seat of the Bulgarians, a day’s journey west of Monastir, was invited by Charles Thopia, lord of Durazzo, to aid him in his war against Balsa, the most powerful Frankish prince of Albania. Khaïreddin was glad of the opportunity afforded by this overture. He crossed the mountains to Elbasan, and then turned southward to meet Balsa. The first battle of the Osmanlis in Albania was fought in the salt-wastes of Savra, on the left bank of the river Devol. The Osmanlis faced fighting men who were fully their equals in courage, in resourcefulness, in strength, and in willingness to engage in a hand-to-hand struggle to death. The issue was long in doubt, and the victory costly. Balsa and his ally and guest, Ivanitch, son of krai Vukasin, were killed.[375] The Osmanlis gained one important result from this battle. Albanian renegades joined their army in great numbers.[376] From that day to this the Albanian element in the Ottoman army, especially among its officers, has been a source of strength which cannot be over-estimated. It is doubtful if the Osmanlis withdrew from Albania, even temporarily, after the battle of Savra; for in 1388 the princess of Valona (Avlona) was so hard pressed by the Osmanlis that she put her domains under the protection of Venice.[377] In northern Albania, the invaders captured Croia and Scutari in 1386. Scutari was given back by Murad in exchange for the addition of a member of the ruling family of Zenta to his harem. From Croia, also, the Osmanlis withdrew. Murad did not want to excite and alarm Venice at the moment when Philippe de Mézières was preaching so vigorously and successfully a new crusade.[378]