An effective army and a large special built cannon to blow down the city walls. The bigger question is why after WW One, was Turkey allowed to keep it?
Yes, also the reason the country Byzantines, esp. in Cilicia, worked with the Turks against the 2nd Crusade. The Crusade would pass, but the good offices of the Turks, living in highland Anatolya, were necessary to prevent raiding that would resume when they left.
Funny, I was talking on X about this today. Certainly it should be the first target of any Crusade. Also, Europe should really end at the Bosphorus.
Good causus belli: Erdogan is using Hagia Sophia as a Mosque again.
After all, it's historically Greek, as is the coast of Turkey. What gave the Turks the right to take it?
And much of the Turkish population is genetically Greek (and Armenian, further east).
https://pjmedia.com/robert-spencer/2021/06/10/turks-enraged-as-ancestry-com-reveals-the-truth-most-of-them-are-greeks-n1453554
Their ancestors were pressured to speak Turkish and convert to Islam. Maybe a lot of them would be happy to recover their true heritage.