Fatih was aware of the fact that, the capture of Istanbul would brought a commercial and cultural richness to his Empire. Moreover, he would took control of the straits and the Black Sea. The famous engineers of the time, as Musluhiddin and Saruca Sekban designed new cannons. Those huge cannons had a great role during the conquest.

On the European side, about five miles above the city, where the channel is the narrowest, Sultan Mehmet raised a fortress opposite to one on the Asiatic side which had been erected by his predecessor Yildirim Bayezid. These fortifications were called the castles of Europe (Roumelia) and Asia (Anatolia). They were intended to command the intervening waters and to secure a point constituted by nature on the high road for both continents.

During those preparations, the peace treaties with Wallachia and Serbia were renovated and a peace treaty was signed with the Hungarians.

The Byzantine Empire was preparing as well. They were stored food for a possible long term siege and they were restoring the city walls. The Byzantine Emperor Constantine heard of the rise of massive towers in his neighbourhood and his anxiety had increased. He asked for the help of the Christian World, but the Pope desired to unify the Orthodox and the Catholic Churches together and the struggle between two Churches had been raised, the Emperor had to forgot a new Crusade.