There were numerous sieges of Constantinople (modern Istanbul) during the history of the Byzantine Empire. Two sieges resulted in the capture of Constantinople: in 1204 by crusaders, and in 1453 by the Ottoman Empire under Mehmed II.
The first Siege of Constantinople began in 1203 during the Fourth Crusade. The crusaders had promised to pay Venice to transport their their fleet of ships to Egypt, but were unable to do so. Their leader, Boniface of Montferrat, met with Alexius Angelus, son of the Byzantine Emperor Isaac II, who had fled to the court of Philip of Swabia. Alexius agreed to pay the debt if the crusaders helped his family return to the throne of the Byzantine Empire. After capturing Zara against the wishes of Pope Innocent III, the crusaders began the siege of Constantinople in 1203, and captured the city in 1204. Alexius Angelus was proclaimed Emperor Alexius IV and his father Isaac II was restored as co-emperor.
The final Siege of Constantinople happened in 1453. The Ottoman Empire had conquered almost all of the Byzantine Empire's territory, leaving Constantinople as a small tributary state. Ottoman Emperor Mehmed II besieged Constantinople with his main weapon — a huge cannon called the Basilic — and 100 000 soldiers. The Basilic could not fire continuously — there was a three hour interval between firing. The cannon collapsed after three weeks because it could not support its own weight. Surrounding Constantinople was 14 miles of walls — the only weak point in the walls was in the Blachernae area, where the crusaders penetrated them 249 years earlier.
When the siege began the Byzantines were able to repair the walls most of the time; however a few wall sections broke and the Byzantines had to drive the Turks back. The walls eventually were undermined, but the Byzantines were able to capture a Turkish engineer, who (after he was tortured) told them the locations of all the mines and the Byzantines countermined all the tunnels.
Mehmed stopped the siege for a time and said he would leave if the Byzantines paid him a massive sum of money which they could not pay. So the siege began again on May 28 and the Turks began assaulting the Blacernae walls and found the Kalkoporta gate was left open and went in. Obviously the gate had been inaccessible due to rubble and other things piled up there from the city, but was easy to get in from outside, and so the Ottomans quickly entered and killed the last Byzantine emperor, Constantine XI.
