In 1379, as already mentioned, John and his son Manuel, who had been captured and imprisoned by his grandson Andronicus, escaped to Scutari and took refuge with Bajazed, the son of Murad. The sultan, after assuring himself that the inhabitants of Constantinople preferred Manuel to Andronicus, made a bargain with John and his son by which, in return for aid in restoring them, the empire should pay a large annual tribute, furnish a contingent of twelve thousand soldiers, and surrender to him Philadelphia, the last remaining city in Asia Minor which still acknowledged the rule of Constantinople. John and Manuel entered Constantinople by the Adrianople Gate, and Andronicus escaped across the Golden Horn to the Genoese in Galata. Much as the two emperors may have regretted their bargain, Murad held them to it, and they, Christian emperors, marched to Philadelphia, in order to compel their own subjects to open its gates to the Turks. Everywhere the Moslem flood was becoming irresistible. The sultan of Bagdad, in 1376, invaded Armenia and took prisoners both its king and queen; at the other extreme of the empire the Turks were in Epirus and were holding their own in many parts of Morea. The Knights-Hospitallers surrendered Patras to them in order to purchase the release of their Grand Master. One of the few strongholds in Thrace which Murad had not hitherto obtained was Apollonia, the present Sissipoli, which, partly built on an island in the Black Sea and in an otherwise strong position, had so far avoided capture. It was taken, however, by Murad in 1383, and, as usual, its garrison was cruelly massacred. In 1385, Murad captured Sofia, and then sent two armies, one to take possession of Cavalla and other places on the north shore of the Aegean, and the other to capture Monastir and various towns in Macedonia. In the same year a Turkish army took Belgrade and pushed on to Scutari in Albania, taking possession of it and of other strongholds. In 1387, after a siege lasting four years, Salonica was captured. The Serbians, by their defeat at Belgrade and elsewhere, were compelled to become the vassals of Murad, and, following his usual custom, the sultan compelled their kral in 1381 to send two thousand men to aid him in subduing a revolt of his brother-in-law, the emir, in Caramania, the ancient Cilicia. Many subjects of the empire had to render like military service. [Sidenote: First battle of Cossovo-pol, 1889.]