The conquest of Istanbul was a necessity for the Ottoman Empire. When Sultan Mehmed II the Conqueror succeeded to the throne, Ottoman borders have gone as far as Danube in Europe. It was very important for the Ottoman Empire to connect Anatolia to Rumelia. The army had to pass form Anatolia to Rumelia safely in any case of a need.

In the past years Ottomans had suffered a lot as they didn’t have domination in the Bosporus. The best example for this was seen during the Battle of Varna. Sultan Murat II’s army who set off from Bursa couldn’t pass the Dardanelles and reach the Balkans as Bosporus had been shut down by Papa’s navy. So Murat II’s army had to take a long way from the Black Sea and had to give one duka gold for each soldier to the Genoese ships.

Meanwhile Istanbul and Galata that were left as an island within the boundaries of the Ottoman Empire created negative impacts for the country’s economy. In order to conquer Istanbul, first the Bosporus had to be taken under control. The ships coming from the Black Sea constituted Istanbul’s source of living. It would be very easy to transfer people and goods from Anatolia to the different regions of the empire after controlling Bosporus.

The Anatolian Fortress that was constructed by Yıldırım Beyazıt was insufficient to ensure the control of the Bosporus Strait. For this reason a fortress had to be constructed in the opposite side. Also to increase the power of this fortress a strong navy was needed. The ships firstly transferred the necessary materials from Anatolia to Rumelia.

Yenihisar was going to be constructed in the place of the bridge that was constructed by Iranian emperor Dara in order to cross his army from Anatolia to Rumelia. Sultan mehmet II the conqueror sent orders all over the empire for the preparations. At the beginning of spring around 700-800 masters and 3000-4000 workers along with the all materials were ready for the construction.

The construction of the Rumeli Fortress began in 26 March 1452. Sultan mehmet II the conqueror along with Halil and Zağanos pashas arrived to the construction site and ordered them to construct a castle in the area.

The construction started with 700-800 masters along with 3000-4000 workers. Each day the masters had to finish two lines of walls. Faith Sultan Mehmet who also worked at the construction site and encouraged the workers. The fortress was completed in 5 months.

Also wooden houses, mosques, cisterns, material stores were built inside the Fortress for the garrison. Firuz Ağa with 400 janissaries was charged with the protection of the fortress. The passage of the ships from the Bosporus strait that didn’t get permission form the ottomans was blocked. During the construction of the fortress many small conflicts arouse. The Byzantine emperor despaired of Europe ordered the construction of the gate of the city walls for defense.

Sultan mehmet II the conqueror stayed in the region until the construction was completed. Later on he came to Istanbul with 10 000 troops. He examined all the walls, military positions and the way of offensive. In order to conquer Istanbul a strong navy and artillery to demolish the walls were needed.

The Navy commander Baltaoğlu Süleyman was tasked with gathering the fleets appropriate for the targets and the Gelibolu yards. The number of warships that was prepared in a short time was enough while their war quality was low. These warships couldn’t cope with the Venetian galleons due to their small sizes.

Sultan Mehmed II the Conqueror ordered his artilleryman Saruca Usta and architect Muslihiddin to make big artillaries to destroy the city walls. In the meantime, an artillery blacksmith called Orban who came from Hungary and started working for the Byzantine Empire left Istanbul and went to Edirne and started serving for Sultan Mehmet II the conqueror. Orban who worked together with Saruca Usta and architect Muslihiddin made giant artilleries in Edirne that could easily destroy the city walls.

These artilleries were first placed to Boğazkesen and Güzelce Hisar that controlled the Bosporus. They achieved great success in preventing the Venetian ships that were carrying materials from Black Sea to Byzantium.Even some of the ships were sunken after which the venetians got aware of the fact that the Bosporus was under the control of the Ottomans.

This success pleased Sultan Mehmed II the Conqueror very much. Now it was time to get prepared for making artilleries to destroy the city walls of Istanbul. In the first 3 months of 1453 giant artilleries even stronger then the previous ones were made. These artilleries having a diameter of 85 centimeters and weighted 600 kilograms could shoot granite cannon balls and were pulled by 50 pairs of oxen.

Besides, craftsman that would dig sewer under the walls were brought from Serbia and Anatolia. Sultan Mehmed II the Conqueror, who assured the safety of the Bosporus, signed new agreement with their allies and kept on 100 000 soldiers stand by for any possible attack, could now easily siege Istanbul.

Byzantine Emperor XI. Constantine spent the first months of 1453 with was preparations. The walls were strengthened, ditches were cleaned, doors were built, additional gates were built and food for six months was stored. Also the Byzantine emperor sent ambassadors to the European states and asked them for help.

Only Venice and Genoa considered the call as Istanbul’s conquest by the Ottomans threatened their commercial interests. With the support of the allies, the Byzantine army reached twenty thousand. The defense of the Golden Horn was undertaken by the Venetians. Iron chains were stretched at the entrance of the Golden Horn.

At that time the Ottoman navy was newly established and naturally wasn’t experienced at sea warfare. The Venetians were more experienced at sea warfare compared to ottomans which was going to appear in the forthcoming days.

Even though the historians said the various figures about the exact number of the Ottoman army, the accepted figures were 12 000 infantries, 8 000 cavalry horseman, 2000 artilleryman and armourer, 40.000 Anatolian and Rumelian soldiers and 82 000 combatant forces.

Also Karaman, Tekeli, Aydın, Hinge, Saruhan, and İsfendiyaroğulları seigniorial participated to the siege with their corteges. Upon the request of Sultan Mehmed II the Conqueror, the Serbian king Broncovitz sent an armored cavalry troop.

The Ottomans didn’t give importance to the navy for many years. They had suffered the difficulties of this ignorance in the past years. And finally Sultan Mehmed II the Conqueror realized that it wasn’t possible to conquer Istanbul with a strong navy. For that reason he wanted to strengthen the navy. Under the command of Baltaoğlu Süleyman Bey 12 galliots, 70 warchips and 20 small ships were added to the Ottoman navy.

The Ottoman army was now ready to conquer Istanbul.