Anyways after Sultan Mehmed keeled over in 1421, his brother Mustafa was released by Emperor Manuel II; his freedom guaranteed by promising to return a few cities to the Byzantines. He captured Gelibolu, the fort which controls the strait of Çanakkale (Dardanelles) and after capturing Edirne the European capital of the empire, he began ruling in Roumeli. He proved that he was Beyazid's son and gained the support of the governors there. Instead of sending the army across the strait of Bosphorus (and having his soldiers desert him), he decides to leave his nephew with the Ottoman holdings in Anatolia and concentrates on eliminating all possible threats to his rule on the European side, namely like partisans like Mihaloğlu, and untrustworthy allies.
How do things proceed on? I can see Mustafa going back on his promise to Manuel II and consider besieging Constantinople. The Byzantines were low on funds and soldiers and barely made it out victorious against the last siege which was in 1422. He might as well try seizing Constantinople then. Tell me of your opinions?