In 1204, the crusaders were attacking Constantinople, the enormous walled capital city of the Byzantine empire (the Byzantines had become an enemy of the defense of Europe). The following is a quote from the excellent book, The New Concise History of the Crusades:
The crusading army boarded its vessels and attacked Constantinople again on April 12, 1204. The battle raged most of the day with the crusaders gaining very little. By the early afternoon, they had captured a few towers on the harbor walls, but they were so surrounded by Byzantine forces that they dared not advance farther. A small group of about ten knights and sixty sergeants led by Peter of Amiens landed on a narrow strip of ground outside the wall near the shore. There they discovered a postern gate [a small, rear gate] that the defenders had walled up. They fell upon it zealously with whatever implements they had on hand. It was hard going and dangerous, for from above the Greeks (Byzantines) rained down on them immense stones and boiling pitch, which they deflected with their shields. At last, they succeeded in making a small hole. Peering through, they saw a huge crowd of soldiers inside. Surely, it was suicide to enter. One man, an armed priest named Alleumes of Clari, insisted on the honor of being the first to enter Constantinople. No amount of pleading from his comrades would dissuade him. His brother, Robert of Clari, was particularly upset and even tried to prevent him from crawling through the whole by grabbing his legs. It was no use. Alleumes scrambled through to the other side, where he was faced with an armed multitude.
Then an amazing thing happened. With enormous confidence, Alleumes drew his sword and ran toward the Greek troups. They scattered. Once again, the poorly trained Byzantine troops proved themselves unwilling to fight unless the danger to themselves was minuscule. Because most of them were provincials [that is, they were not from Constantinople], they saw no reason to risk their lives for the sake of the capital. Alleumes called to his companions, who now crawled through the hole, drew their swords, and kept their backs to the wall. When Greek troops at other locations saw the flight of those stationed near the walled gate, they panicked and abandoned their positions along the walls. Soon there was a snowball effect as the imperial army abandoned the length of the fortifications in a mad dash to escape the city — all because of the entry of fewer than a hundred crusaders. Mourtzouphlus [the emperor of Constantinople], who watched the debacle with horror, spurred his horse and single-handedly charged the small party. When he saw that they would not retreat, he stopped and returned to his command station. With no one to stop them, Alleumes and his comrades opened the city gates, and the entire crusading army swarmed in.
That evening, the crusaders camped in the open region in the north city that had been burned by the Venetians nine months earlier. Strict orders were given to the host that no one was to venture farther into the city itself. In the winding streets and back alleys of the megalopolis, the mobs could pick off crusaders easily. They resolved to call for the surrender of the city on the morrow. If that was refused, they would begin burning the place down.
With the imperial army in full retreat, Mourtzouphlus could rely only on the Varangian Guard [Viking mercenaries]. It was not enough. Desperately, he rode through the streets exhorting the people of Constantinople to rise up and defend their city from the invading westerners. No one would listen. From their perspective, it was not their city that was at stake, but Mourtzouphlus's crown. They would not suffer further damage to the capital to save his reign [he wasn't very popular]. Instead, they made plans to offer the city peacefully to Boniface of Mohtferrat [a crusader], who they assumed would be the new emperor. Once he had the imperial throne, they expected that Boniface would restrain his army, since it was not in his interest to have his city destroyed. At least that is the way it had worked in all the other armed coups in Constantinople's long history. Without the support of the people or the army, Mourtzouphlus fled the city.