Why is the conquest of Constantinople so important to Turks? What made Constantinople so great? Who made Constantinople so great that Turks still celebrate to this day about conquering it?
The only reason 1453 matters so much to Turkish nationalism is that Constantinople was already one of the greatest cities in history long before it was ever taken. The people who made it great were the Orthodox Christian Graeco-Romans whose living continuation is the modern Greeks, and they were the ones who raised its walls and churches and palaces and filled it with learning and wealth and gave it the prestige that made it worth conquering in the first place.
No other nation builds so much of its national identity around the capture of a single city it did not even build, which tells you that what is really being celebrated is stealing something good they couldn't make.
So when the conquest gets celebrated, what is really being celebrated has nothing to do with creating anything good or with defending anything from something evil, because the city was already built and it was good. All that happened in 1453 was that someone else's masterpiece changed hands while the people who stole it started talking as if its greatness had always been theirs. It is like a man who steals the most beautiful house in the world while despising the person who built it, and then spends the rest of his life inventing reasons for why he supposedly improved the place, as if repainting the front door makes him the architect.
And the copes only get more desperate from there, because the people they despise never actually disappeared. The Orthodox Christian Graeco-Romans carried the same faith and the same Greek language and the same culture straight through the medieval and Ottoman centuries and became the modern Greeks, and that unbroken continuity drives Turkish nationalism crazy and makes Turks insecure. So Turks keep inventing stories about how the modern Greeks are some brand new people who popped into existence out of nowhere in 1821 with no real link to the medieval and Ottoman era Greeks who shared their exact language, religion, and culture. They brag about stealing the city, they hate the people who built it, and then they pretend those same people's living continuation are total strangers to them.
Their favorite excuse is that the city was in decline by the time they took it and that they were the ones who saved it, which conveniently skips over the fact that the Turks were the reason it declined in the first place. It was the Seljuks and then the Ottomans who tore Anatolia away from the empire, and Anatolia was the heartland that fed Constantinople its people and its soldiers and its food and its wealth, so once that was gone the city was cut off from the very thing that had kept it alive for a thousand years. They strangled it slowly over centuries and then stood over the body claiming they were the ones who brought it back.
There is another cope sitting underneath all of it, which is the idea that Greece had nothing to do with Anatolia, as if the two were always separate worlds. The truth is that the people of Greece and Anatolia and Cyprus were one and the same in the medieval period. They were Greeks in the Hellenistic age and Greeks again in the early Roman empire, and they were all Romanized together until they became the Graeco-Romans of the medieval world, one people sharing one language, one culture, one identity and one faith across. Greece and Cyprus today simply happen to be the last of that Graeco-Roman people still standing, after the ones in Anatolia were converted, mixed with alien Turks and killed and finally 99% expelled in the 20th century. The east Romans never saw any of these lands as separate nations in the first place, because they had a single name for the whole thing, Rhomania (Ρωμανία), meaning the land of the Romans.
Make that make sense.
Anatolia, Greece and Cyprus were equally Rhomania when the Turks arrived.
Fireworks to celebrate the conquest of Istanbul on its 573rd anniversary