Siege of Constantinople ( 1203 ) The Siege of Constantinople in 1203 was a Crusader siege of the capital of the Byzantine Empire , in support of the deposed emperor Isaac II Angelos and his son Alexios IV Angelos . The siege . To take the city by force , the Crusaders first needed to cross the Bosphorus . About 200 ships , horse transports and galleys would undertake to deliver the crusading army across the narrow strait , where Alexius III had lined up the Byzantine army in battle formation along the shore , north of the suburb of Galata . The Crusaders ' knights charged straight out of the horse transports , and the Byzantine army fled south . The Crusaders followed south , and attacked the Tower of Galata , which held one end of the chain that blocked access to the Golden Horn . As they laid siege to the Tower , the Greeks counterattacked with some initial success . However , when the Crusaders rallied and the Greeks retreated to the Tower , the Crusaders were able to follow the soldiers through the Gate , and the Tower surrendered . The Golden Horn now lay open to the Crusaders , and the Venetian fleet entered . On July 11 , the Crusaders took positions opposite the Blachernae palace on the northwest corner of the city . Alexios IV was paraded outside the walls , but the citizens were apathetic , as Alexios III , though a usurper and illegitimate in the eyes of the westerners , was an acceptable emperor for the Byzantine citizens . The siege began in earnest on July 17 , with four divisions attacking the land walls , while the Venetian fleet attacked the sea walls from the Golden Horn . The Venetians took a section of the wall of about 25 towers , while the Varangian guard held off the Crusaders on the land wall . The Varangians shifted to meet the new threat , and the Venetians retreated under the screen of fire . The fire lasted for 3 days and destroyed about of the city , leaving 20 , 000 people homeless . Alexius III finally took offensive action , and led 17 divisions from the St . Romanus Gate , vastly outnumbering the Crusaders . Alexius III ' s army of about 8 , 500 men faced the Crusader ' s 7 divisions ( about 3 , 500 men ) , but his courage failed , and the Byzantine army returned to the city without a fight . On July 18 , 1203 the Crusaders launched an assault on the city , and Alexios III immediately fled into Thrace . The next morning , the Crusaders were surprised to find that the citizens had released Isaac II from prison and proclaimed him emperor , despite the fact that he had been blinded to make him ineligible to rule . The Crusaders forced Isaac II to proclaim his son Alexios IV co - emperor on August 1 , effectively ending the siege . After the 1203 siege . Following the end of the first siege of Constantinople in 1203 , on 1 August 1203 , the pro - Crusader Alexios Angelos was crowned Emperor Alexios IV of the Byzantine Empire , who then tried to stabilize the city . But riots between anti - Crusader Greeks and pro - Crusader Latins broke out later that month and lasted until November , during which most of the populace began to turn against Emperor Alexios IV . On 25 January 1204 , the death of co - Emperor Isaac II set off rioting in Constantinople in which the people deposed Alexios IV , who turned to the Crusaders for help but was imprisoned by the imperial chamberlain , Alexios Doukas , who declared himself Emperor Alexios V on 5 February . Emperor Alexios V then attempted to negotiate with the Crusaders for a withdrawal from Byzantine territory , but they refused to abandon their old treaty with Alexios IV . When Alexios V ordered Alexios IV ' s execution on 8 February , the Crusaders declared war on Alexios V . In March 1204 , the Crusader and Venetian leadership decided on the outright conquest of Constantinople , and drew up a formal agreement to divide the Byzantine Empire between them . By the end of that month , the combined Crusader armies had begun the Siege of Constantinople ( 1204 ) as Emperor Alexios V began to strengthen the city ' s defences while conducting more active operations outside the city .