In 1453, an invading Ottoman army led by Sultan Mehmed II besieged and conquered Constantinople, capital of Byzantium. The defeat of the army of Constantine XI marked the final end of the Roman Empire. Though both the besiegers and the besieged used conventional incendiary weapons based on Greek Fire, this conflict is notable in that the Ottomans introduced a super-large siege-cannon that used gunpowder to destroy the walls of the city with missiles. The Arabs, who had acquired knowledge of gunpowder after 1240, had succeeded in increasing its explosive capabilities by improving the quality of the saltpetre (potassium nitrate (KNO3).