As a result of the Ottoman victories in Roumelia and their advance through Bosnia and Albania, the Christian world had panicked. With Papa’s provocation the Hungarian King Sigismund commanded the Crusade composed of many European countries as France, England and Germany in May 1376.

The aim of this allience was to protect Constantinople, the city which had been under siege for five years. Crusaders besieged the Castle of Nigbolu located on the Danube's shore. The Ottoman commander Dogan defended the castle successfully until the Ottoman Army commanded by Sultan Bayezid had arrived. A very bloody battle had been fought (1396). Crusaders were defeated in the Battle of Nigbolu. At the end of the battle, the lands Crusaders had captured, had taken back by the Ottomans. Bulgarian Kingdom was ended and raids were organised through Hungary. After this victory the Crusades did not attack the Ottomans for fifty years. The Caliph of Abbasids called Bayezid as “The Sultan of Anatolia”.

After Nigbolu Battle, Constantinople was besieged for the third time. The Anadoluhisari (fortress located in Anatolian site of Istanbul) was built. However, lack of a powerful navy and huge cannons hindered the conquest. Therefore, Bayezid had begun to establish a powerful Ottoman Navy. He thought that Constantinople would have been surrender by the siege. But when the danger of Timur appeared, a treaty was signed with the Byzantine Empire and the siege was withdrew. By means of this treaty, a mosque in Sirkeci in Constantinople, an Islamic Court and a Turkish quarter were built.

In 1398, Karaman state and the Turkish states in the Black sea were conquered. One year later, Dulgadirogullari State was ended. Bayezid, also, battled against the Geneose Colony in Galata in Constantiople.

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