Meanwhile, despite some probing attacks, the Ottoman fleet under Suleiman Baltoghlu could not enter the Golden Horn due to the boom the Byzantines had laid across the entrance, and although one of its main tasks was to prevent any ships from outside from entering the Golden Horn, on 20 April a small flotilla of four Christian ships managed to slip in after some heavy fighting, an event which strengthened the morale of the defenders and caused embarrassment to the Sultan. Baltoghlu's life was spared after his subordinates testified to his brave yet fruitless efforts to Mehmed. To circumvent the boom, Mehmed ordered the construction of a road of greased logs across Galata on the north side of the Golden Horn, and rolled his ships across on 22 April. This seriously threatened the flow of supplies from Genovese ships from the - nominally neutral - colony of Pera and demoralized the Byzantine defenders. On the night of 28 April, an attempt was made to destroy the Ottoman ships already in the Golden Horn using fire ships, but the Ottomans had been warned in advance and forced the Christians to retreat with heavy losses. From then on, the defenders were forced to disperse part of their forces to the Golden Horn walls, causing defense in other sections of the walls to weaken.
The Turks had made several frontal assaults on the land wall, but were always repelled with heavy losses. From mid-May to 25 May, the Ottomans sought to break through the walls by constructing underground tunnels in an effort to mine them. Many of the sappers were Serbians sent from Novo Brdo by the Serbian Despot. They were placed under the rule of Zaganos Pasha. However, the Byzantines employed an engineer named Johannes Grant (who was said to be German but was probably Scottish), who had countermines dug, allowing Byzantine troops to enter the mines and kill the Turkish workers. The Byzantines intercepted the first Serbian tunnel on the night of 16 May. Subsequent tunneling efforts were interrupted on 21, 23, and 25 May, destroying them with Greek fire and vigorous combat. On 23 May, the Byzantines captured and tortured two Turkish officers, who revealed the location of all the Turkish tunnels, which were then destroyed.
Mehmed offered to lift the siege if they gave him the city. When this was declined, Mehmed planned to overpower the walls by sheer force, knowing that the weak Byzantine defense would be worn out before he ran out of troops. Around this time, Mehmed had a final council with his senior officers. Here he encountered some resistance; one of his Viziers, the veteran Halil Pasha, who had always disapproved of Mehmed's plans to conquer the city, now admonished him to abandon the siege in the face of recent adversity. Halil was overruled by Zaganos Pasha, who insisted on an immediate attack, an advice which the Sultan was glad to follow. Suspected of having been bribed by the Byzantines, Halil Pasha was put to death later that year.
On May 22, 1453, the moon, symbol of Constantinople, rose in dark eclipse, fulfilling a prophecy on the city's demise. Four days later, the whole city was blotted out by a thick fog, a condition unknown in that part of the world in May. When the fog lifted that evening, a strange light was seen playing about the dome of the Hagia Sophia, and from the city walls lights were seen in the countryside to the West, far behind the Turkish camp. The light around the dome was interpreted by some as the Holy Spirit departing from the Cathedral, while there was a distant hope that the lights were the campfires of the troops of John Hunyadi who had come to relieve the city.
The following day a small Venetian ship of 12 entered the Capital and reported to the Emperor that no Venetian relief fleet was on its way after having searched the Aegean. Nonetheless the Emperor was able to receive the aid of the 12 in the defense of the city.