Early in his reign Mahomet recognized the strategic value of Constantinople. It became the keystone of his Empire. He transferred the seat of his government to it from Adrianople. He fortified the Dardanelles by the erection of two castles on either side of it near to Sestos and Abydos, each with thirty guns, which commanded the Straits. This secured his capital from attack. It prevented the entrance of a hostile fleet into the Sea of Marmora and the Black Sea. He added greatly to his navy, and made it superior to that of any other single Power in the Mediterranean. It gave him absolute supremacy in the Black Sea and the Sea of Marmora. The possessions of the Genoese in the Black Sea were at his mercy. He sent a flotilla of small vessels up the Danube to assist in the siege of Belgrade. Throughout all his campaigns Mahomet exhibited perfidy and cruelty on a scale almost without precedent. Princes, generals, and armies, who capitulated on the promises of safety of life and respect of property, were put to death without compunction, in gross breach of faith. The inhabitants of cities were sold into slavery or transferred forcibly to Turkish dominions, in total disregard of solemn pledges. A notable case of this kind was that of Bosnia, where the final victory was achieved by the Ottoman Grand Vizier, in command of one of the armies engaged, under the supreme command of the Sultan. The Prince of Bosnia and his army capitulated on the distinct engagement in writing that their lives would be spared. Mahomet was full of wrath at this concession. It was his deliberate policy to extinguish by death the family of any reigning prince whom he vanquished in war. He consulted on the point the Mufti, with doubtless a strong hint as to what the answer should be. The Mufti issued a _fetva_ which declared that no treaty of this kind with an infidel was binding on the Sultan. The holy man went so far as to offer himself to act as executioner. When the Bosnian king was summoned to the presence of the Sultan, and came before him trembling, with the treaty of capitulation in his hand, the Mufti himself struck off his head in the presence of the Sultan, exclaiming that it was a good deed to put an end to an infidel. The _fetva_ in this case formed a precedent for numerous similar cases. The whole of the royal family of Comnenus, the Emperor of Trebizond, who, without a fight, surrendered his kingdom to Mahomet, upon the promise of life and private property to himself and his family, were put to death a few weeks later in Constantinople on the most flimsy pretence.