This article discussed the background of the Byzantine aspect of the siege and conquest of Istanbul. The examination of this event takes the psychological situation of the Byzantine people into account. In this context, it is important to refer to and observe contemporary Byzantine sources, including the records of Francis -the man closest to the emperor- who was in the city during the siege. In addition, it is crucial to consider the information available from the Venetian ship doctor who coincidentally happened to be in the city at the time and participated in the defense using Nicolo Barbaro's observations reflecting the psychological situation in the city. In its evaluation of the conquest of Istanbul from the Byzantine aspect, this article takes into consideration these Byzantine records, paying special attention to the firsthand observations of the aforementioned eyewitnesses.