The Turks made numerous frontal assaults on the wall, but were 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 sap 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 countertunnels dug, allowing Byzantine troops to enter the tunnels and kill the Turkish workers. The Byzantines intercepted the first Serbian tunnel on the night of 16 May. Subsequent tunneling efforts were intercepted on 21, 23, and 25 May. The Christian defenders of Constantinople destroyed the tunnels 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. [12]
Mehmed offered to raise the siege for an astronomical tribute that he knew the city would be unable to pay. When this was declined, Mehmed planned to overpower the walls by sheer force, knowing that the Byzantine defenders would be worn out before he ran out of troops.