Constantine appealed to Western Europe for help, but [[Nicholas V of Rome|Pope Nicholas V]] was unwilling to support the Empire. Ever since the [[Great Schism|mutual excommunication]] of the [[Orthodox Church|Orthodox]] and [[Roman Catholic Church|Roman Catholic]] churches in 1054, the Roman Catholic West had been trying to re-integrate the East; the West now used this as a negotiating tactic, promising to send help if the Byzantines brought their church back into [[full communion|communion]] with Rome. Attempts had been made to do this after the [[Council of Florence]] and the Council of Basel, but the Orthodox population refused to support it. Pope Nicholas and many other western leaders made the decision not to support the Empire, although some troops did arrive from the city states of northern Italy.

The Ottomans employed a Hungarian engineer called Urban who was a specialist in the construction of cannons, which were still relatively new weapons. He built an enormous cannon, nearly twenty-seven feet (more than 8 m) in length and 2.5 feet (about 75 cm) in diameter, which could fire a 1200 lb (544 kg) ball as far as one mile. It was dubbed "the Basilic". Although the Byzantines also had cannons, they were much smaller and their recoil tended to damage their own walls. Urban's cannon had several drawbacks, however. It could hardly hit anything, not even as large as Constantinople; it took three hours to reload; the cannon balls were in very short supply; and the cannon collapsed under its own recoil after six weeks.