Venetian and French crusader forces, diverted from their original goal of Jerusalem, scaled the sea walls of Constantinople using ships' ladders and captured towers along the Golden Horn. Emperor Alexios III Angelos fled the city in the night, abandoning his throne, and the crusaders restored the deposed Isaac II Angelos and his son Alexios IV to power. The episode was a prelude to the catastrophic sack of the city the following year.
The Fourth Crusade had already gone badly astray by the time it reached Constantinople. Venice, financing the expedition, redirected the crusaders first to sack the Christian city of Zara on the Adriatic, then to Constantinople itself, ostensibly to restore a deposed Byzantine prince, Alexios Angelos, who promised vast payment and troops for the Holy Land in return.
On July 17, 1203 (with the assault beginning the day prior), Venetian doge Enrico Dandolo, elderly and nearly blind, personally led an amphibious assault, standing at the prow of his galley with the banner of St. Mark as ladders were thrown against the sea walls. Crusader knights broke through weakly defended towers while a fire, possibly set to cover a retreat, spread through the city. Emperor Alexios III, offered no serious resistance from his mercenary Varangian Guard, slipped away with treasure and his daughter, leaving the throne empty.
The crusaders got their puppet emperor, but Alexios IV could not deliver the promised gold, and tensions festered for months. The following April, the crusaders returned in fury and sacked Constantinople outright, an act of destruction against a fellow Christian empire that permanently crippled Byzantium and hastened its eventual fall to the Ottomans two and a half centuries later.