Ottoman Turks became interested in Istanbul during the 14th century. All residential areas of today’s Istanbul besides Surici (inside the city walls) became the territory of the Ottoman Empire way before the conquest. Ottomans also interfered with the internal affairs of Byzantine Empire during the same perion of time. Manoeuvring aroud Istanbul continuously, they were getting prepared for the final cut.
Ottoman armies reached the gates of Istanbul in 1340, but they did not lay a siege. A strong Christian alliance stopped the campaign which was initiated by Sultan Murad I at Catalca.
The first big siege aimed to conquer Istanbul was realised by Sultan Yildirim Bayezid. However, his army did not enter the city as a result of the agreement he made with the emperor. Sultan Yildirim Bayezid influenced Istanbul afterwards,too. He managed to establish a Turkish square, a mosque and a court which gave trial to Turks. He would support the emperors who would take care of Ottoman interests. This was one of the most important factors that affected the conquest of Istanbul by Turks.
The last siege attempt during the reign of Sultan yildirim Bayezid occured in 1400. But the Timur problem interfered with this. The siege led by Musa Celebi, son of Sultan Yildirim Bayezid, in 1411 was also unsuccessful. The Emperor, fearful of the success of the Ottoman army, obtained the support of Mehmet Celebi, brother of Musa Celebi, from Bursa and the siege was lifted. Afterwards, during the reign of Celebi Mehmed there were no military campaigns against Istanbul.
The last siege before the conquest occured during the reign of Sultan Murad II. The strategic planning phase of the siege took a long time and it was very powerful strategic plan. But the siege was more difficult than the previous ones. The siege started on June 15, 1422 by 10,000 cavalry men with the blocking of the roads which connected Istanbul to other cities. Emir Sultan, one of the most powerful spiritual leaders of that time came from Bursa and joined the army with his hundreds of dervishes. That positively affected the soldiers. The attack on August 24th, which Emir Sultan also participated in, was very intense, but not enough toconquer the city. the siege was lifted because of the rebellion of Shehzade Mustafa who was the brother of Sultan Murad II. This left the task of the conquest of Istanbul to the son of Sultan Murad II.