It is true that the Byzantine Empire never collapsed – it was conquered from the outside. It wasn't like Soufflé, accompanied by a long whimper and a final loud bang.
But let's assume that the guards at the Kerkoporta gate have been vigilant and closed the door. In vain, Mehmed II hit the walls of Constantinople with his head and finally retreated.
As a reward for his heroic work, Konstantinos XI nominated Giovanni Giustiniani as sebastokratōr under the name Ioannis Iustinianos. He now had an imperial office, not just a mercenary. These two strong and energetic men decided to save what could be saved and worked to unify the imperial lands of Greece. They face a hopeless mission, but they have just survived a hopeless siege.
The first thing they need to do is negotiate some sort of religious agreement with the West. This was an extremely bitter pill for the Greek populace and shouted philiokoue! kiss! It could be heard everywhere, but the amendments of the Church of Rome were enough to appease the fanatical Romans. Both the Patriarch of Constantinople and the Pope of Rome abolished the curse on each other (historically this happened in 1965).
At the same time, the brutality of the Ottoman Turks in Albanian and Serbian soil evoked the mood of the Crusades in the West, especially the Poles and Hungarians were very enthusiastic about it. The Polish king persuaded the Teutonic Knights Hochmeister to fight the Muslim Turks instead of the newly converted Lithuanians. Since the Teutonic Knights had long problems recruiting new crusaders, they gladly accepted the offer. The Knights were now recruiting crusaders in Germanic lands.
A crusader army led by King Kazimierz Jagiello of Poland, King Janos Hunyadi of Hungary and the Grand Master of the Teutonic Knights arrived in Transylvania. Their aim was to seek a battle against the Ottoman Empire in terrain favorable to heavy cavalry. Led by Janos Hunyadi, an experienced and familiar with the Ottoman Empire and its tactics, the army was more disciplined and organized than the army defeated at Varna a decade earlier.
At the same time, the diplomacy of Constantine XI was a success. While the Crusader army kept the Ottoman Turks busy in Wallachia and Moldavia, the Knights of St. John destroyed the Turkish logistics, enabling the Genoese, the Knights of St. John, and the Imperial army to retake Thessaloniki. The Empire once again gained a foothold in Thessalii.
The Crusaders succeeded in retaking Wallachia and Moldavia and gaining a foothold in Serbia. The Sultan, whose army had been depleted by a futile attack on Constantinople and now under attack by the Crusaders, decided to reconcile with the Christian Union. At the same time, the grand masters of the Teutonic Knights were already convinced that the idea of migrating from the Baltic Sea to the Mediterranean (originally represented by Conrad von Junggen in 1390) was a good idea, and the Knights bought the island of Euboia from the Venetians. They brought with them the northern way of shipbuilding, and soon the teutonic gears and warships could be seen on the archipelago.
For decades, this has remained the status quo. At the same time, the Empire hoarded money and money to re-equip its army and train its soldiers. Eventually, after the death of Mehmed II in 1483, the Empire fought back. With the help of the Knights of St. John and the Teutonic Knights, they recaptured Thessalis, Greece, and Attica. The lands between Constantinople and Moriya (Peloponnese) were reunited again. The Ottoman Turks no longer attempted to attack Europe, but concentrated on dismantling Islamic lands in the east under a caliphate and driving the Mongols out of the east. Timur's last land fell in 1503 and Bayezid II was crowned Shah in Iran.
The empire has now successfully rebuilt itself and is made up of most of the lands of modern Greece. This situation has stabilized for nearly 400 years. At Andreas, after the death of the last Paliologues, Charles V, Holy Roman Emperor of the Habsburgs, succeeded to the throne and united the Holy Roman Empire and the Eastern Roman Empire into a personal alliance. After his death, the Habsburgs ascended to the throne.
The Empire served as a buffer state between Western Europe and the Islamic East, surviving through successful diplomacy, alliances, vibrant marriages, and armies. It remained neutral in World War I, but was severely damaged by the Germans in World War II.
Today, empire is one of the territories of the European Union. Despite its military might and cultural tourism superpower, it is unfortunately known for corruption, economic turmoil, and internal turmoil. Today, it is a constitutional monarchy under Emperor Constantinos XIV and a popular destination for tourism. The Arena's international horse racing competitions were televised worldwide, with the first Olympic Games being held in Constantinople in 1896 and again in 1996. Relations with Turkey were not good, but manageable, and both the Ottoman and Habsburg families were interconnected by many marriages.