Urban was a Hungarian entrepreneur who specialized in engineering and gunsmithing. Before the Siege of Constantinople, he had managed to get an audience with Emperor Constantine XI, the head of the Byzantine Empire in Constantinople, and tried to sell his services to the Byzantine Empire. He had plans to build a 13-foot "super-gun," which would be larger than the largest gun ever made.
Constantine was not interested, because he thought Urban asked for too much money, and he thought the huge amount of bronze, a scarce resource, was not warranted to construct just one gun, no matter how big.
Urban, however, had no loyalty to Byzantium, so, when turned down by Constantine, he went to Adrianople, capital of the Ottoman Empire, where new Sultan Mehmed II accepted Urban's proposal, in fact paying him four times what Urban asked. He also promised Urban all the resources he would need.
Twelve weeks later, Urban finished the super-cannon and Mehmed installed it in his new fortress 10 miles away from Constantinople. With this gun, Constantinople was blockaded from trade with Europe through the sea. The super-gun proved its effectiveness, sinking a Venetian merchant craft. After that, Venice and Genoa were forced to make trade agreements with the Ottomans, being uninterested in war with the Ottomans, and needing the lucrative trade routes through Constantinople.
In 1453 the Ottomans blockaded Constantinople and began the siege. Urban had come up with an even bigger gun for Mehmed, more than twice the size of his original super-gun. Its bronze barrel was greater than 26 feet long, 30 inches in diameter and capable of firing 800 pound projectiles at huge ranges. It took 60 oxen to pull, and 200 men to operate. Despite these downsides, and the fact that the cannon could only be fired up seven times a day, Constantinople"s triple walls could not take the amount of hammering the "super-super-gun", as well as the damage Urban's other 30 smaller cannons handed out, which was a key part of the Ottoman Empire's victory.