Hussar_Regimeny,

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You're correct in your understanding of Constantinople's fortifications, and the Crusaders would likely have failed if not for a few key events.

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The first of these was the murder of the Byzantine emperor Alexios IV. Alexios IV had struck a deal with the Crusaders, promising them hundreds of thousands of silver marks. Before they could be paid in full, the radical anti-Crusader nobleman Alexios Doukas managed to imprison and kill Alexios IV. Doukas quickly took the mantle of emperor with the support of the legions. For the Crusaders, this denial of what they considered rightful payment was an unbelievable affront, driving them into a rage.

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Leading up to the Sack of Constantinople, the weather conditions had prevented the Crusaders from penetrating the city's defenses, but a few days into the assault the wind changed very suddenly. This allowed the fuming Crusaders to maneuver their ships to move closer to the Byzantine fortifications than either side anticipated. The movement of the ships along the very edge of the city gave the Catholics a valuable tactical advantage in breaching the defensive walls of the city. The Crusaders themselves ran with the opportunity provided by the shift in the weather, giving them the element of surprise over an unprepared Byzantine army.

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Once the Crusader army breached the walls, about seventy men entered the city and resisted retaliation by creating a 'wall of fire'. Unfortunately for the Byzantines, the fire quickly raged out of control and ravaged the city, forcing the Eastern army to retreat while Crusaders returned to their ships to wait out the blaze. The sudden shift of the battle in favor of the Crusaders caused many of Byzantium's Anglo-Saxon mercenaries, the main defenders of the city, to reconsider their position. After their Byzantine patrons refused to raise their wages accordingly, many deserted or surrendered to the Crusader forces. With the discouraging breach of their walls, the deserting of the Anglo-Saxons, the great fire, and the flight of the emperor in the cover of night, the forces of Byzantium were deeply demoralized. The morning after the fire, the injured, confused, and scattered Byzantines fell to the Crusader army.

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Alongside these events, I think it's important to note the religious aspect of the conflict. The Byzantine empire was on the brink of collapse as it was. The city was suffering economically, the leadership was poor, and constant conflict with foreign powers had greatly depleted the once-great city. In contrast, the Crusaders were riding high and fueled by manic religious passion. They were fighting a Holy War as knights of God's heavenly army. Their religious zeal cannot be understated when analyzing the reasons for the Crusaders' victory. This fanaticism inspired the soldiers as they fought desperately for their salvation. Spurring this on was the recent discovery of a precious relic in the initial breach of Constantinople. The Crusaders had discovered and stolen the Image of Edessa in the conflict, and this was perceived by the Catholic army as a sign from God of his approval and desire for their victory. The recovery of the relic drove the forces into a religious frenzy. Between the religious culture of the Crusader armies and the fortuitous circumstances of the attack, the Catholic army managed to beat the odds and take the Byzantine capital very quickly and brutally.

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Sources:

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Tyerman, Christopher. The World of the Crusades: An Illustrated History. Yale University Press, 2019.

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Hill, Rosalind M. T., trans. The Deeds of the Franks. Oxford: Clarendon Press, 2002.

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Ciggaar, Krijnie Nelly., and H. Teule. East and West in the Crusader States: Context, Contacts, Confrontations. Peeters, 2004.