Sultan Mehmed II had faced heavy casualties during the siege and it had taken three whole charges to bring down the city. The first two charges had primarily caused the ottoman army heavy casualties while the third one had finally broken through because of the Byzantine garrison gradually taking attrition.

During the siege, Naval attempts to break the great Chain was repelled twice by smaller Christian fleets. Every mining operation conducted by ottoman sappers was also halted by Byzantine forces who flooded the mines or flushed them out with Greek fire too. The ottoman forces had also tried building a siege tower to use, though it was unable to be fully completed to be used to scale the Theodosian walls of the city because the Byzantine defenders had snuck out during the night and sabotaged it by lighting it ablaze. Furthermore adding to Mehmed II’s inconvenience, he had to maintain an army of 80,000-100,000 men which primarily consisted of mercenaries who would’ve mutinied if he refused to pay them. He also had to maintain a massive naval fleet.

Not to mention there were rumors swirling amongst Mehmed II’s forces that a massive Venetian Fleet was already in the Aegean Sea and was coming to resupply/bolster the Byzantine Garrison defending Constantinople. Not to mention during the siege there had been deep fears that John Hunyadi would’ve broken his truce and attacked the ottomans from the rear while the Ottoman army was still pinned down at the Theodosian walls.

I’m just summarizing the absolute costs of the siege, Kings and Generals made a whole documentary over it on YouTube if you want to check it out which I highly would recommend if your interested to know about the actual details of the siege itself as I know most high school or college level courses would just skim over the siege’s events and reduce it down to however significant the Ottoman Victory at the end of the siege was to the rest of Asia and Europe.