Murad, having defeated this claimant to his throne, determined to avenge the perfidy of the Emperor Manuel and to put an end to the Greek Empire by the capture of Constantinople. For this purpose he collected an army of veterans. He invested the city, making a long line of great earthworks from the Golden Horn to the Sea of Marmora. From this he bombarded the city walls by cannon, then for the first time used by the Ottoman army, but which were not as yet very effective. He also used movable towers, from which assaults could be made on the walls of the city. He proclaimed that the great wealth of the capital would be the prize of the soldiers if the assault on it were successful. He made a special promise to a band of five hundred Dervishes, who were to lead the assault, that all the nuns in the city would be given to them as concubines. In spite of these great inducements to victory, the assault was unsuccessful. The Greeks defended the walls of the city with the utmost heroism, assisted, it was said, by a timely apparition of the Holy Virgin, which stimulated their efforts and depressed the assailants. Murad would probably have been successful with the overwhelming forces at his disposal if he had persisted in the siege, but he was compelled to raise it by a diversion cleverly contrived by the Greek Emperor. A rival to the Sultan was set up in Asia in another Mustapha, a younger brother of Murad, who had not been put to death in pursuance of the fratricidal policy of his family. This new claimant was supported by the Karamanians and Kermians, and with their aid he defeated an Ottoman army in Asia Minor. Murad found it necessary to abandon the siege of Constantinople, and to transfer his main army to Asia Minor for the purpose of dealing with this danger to his throne. He came to close quarters as quickly as possible with Mustapha’s army, and defeated it. Mustapha was taken prisoner and was hanged at once by his captors, without giving an opportunity to Murad to exercise his clemency in favour of his brother, had he so willed it. Murad then occupied himself by reducing the Karamanian and other Emirs to complete subjection to his Empire. Meanwhile the Emperor Manuel died, and was succeeded by John Palæologus. Murad, in lieu of renewing the siege of Constantinople, was content to make another treaty with the new Emperor, imposing on him a heavy tribute and stripping him of almost every possession beyond the walls of his capital. The Empire thus obtained a reprieve for a few brief years.