The first full-scale OttomanSiege of Constantinople took place in 1422 as a result of the Byzantine EmperorManuel II's attempts to interfere in the succession of Ottoman Sultans, after the death of Mehmed I in 1421. This policy of the Byzantines was often used successfully in weakening their neighbours.
When Murad II emerged as the winning successor to his father, he marched into Byzantine territory. The Turks had acquired their own cannon for the first time by the siege of 1422, "falcons", which were short but wide cannon. The two sides were evenly matched technologically, and the Turks had to build barricades "in order to receive… the stones of the bombards."
The Byzantine defenders won the battle. Contemporary Byzantine tradition ascribed the deliverance of Constantinople to a miraculous intervention by the Theotokos.
In spite of the Byzantine victory, the 'Empire' at this time had in fact been reduced to a few disconnected strips of land besides the city of Constantinople itself. It was also facing grave economic problems and severely lacked soldiers, Pius II promoted the affordable donation of cannon by European monarchs as a means of aid. Any new cannons after the 1422 siege were gifts from European states, and aside from these no other advances were made to the Byzantine arsenal. As such, the next Ottoman leader, Mehmed II, would be successful in 1453.
Byzantine accounts attributed the lifting of the siege to an apparition of the Theotokos upon the city walls, which greatly inspired the defenders.John Kananos records that:
The Romans, although exhausted from fatigue, leapt and were glad… They shouted hymns to the Most Holy Virgin, glorifying her from the depths of their hearts, saying "This is in truth a rich, celebrated, memorable, extraordinary and remarkable miracle worthy of admiration."