Task: Question Generation
Input (English): Passage: On 12 April 1204, the weather conditions finally favoured the crusaders. A strong northern wind aided the Venetian ships in coming close to the walls, and after a short battle approximately seventy crusaders managed to enter the city. Some were able to knock holes in the walls, large enough for only a few knights at a time to crawl through; the Venetians were also successful at scaling the walls from the sea, though there was fighting with the Varangians. The Anglo-Saxon "axe bearers" had been amongst the most effective of the city's defenders, but they now attempted to negotiate higher wages from their Byzantine employers, before dispersing or surrendering. The crusaders captured the Blachernae section of the city in the northwest and used it as a base to attack the rest of the city. While attempting to defend themselves with a wall of fire, however, they burned even more of the city. This second fire left 15,000 people homeless. The crusaders completely took the city on 13 April. The crusaders sacked Constantinople for three days, during which many ancient Greco-Roman and medieval Byzantine works of art were stolen or ruined. Many of the civilian population of the city were killed and their property looted. Despite the threat of excommunication, the crusaders destroyed, defiled and looted the city's churches and monasteries. It was said that the total amount looted from Constantinople was about 900,000 silver marks. The Venetians received 150,000 silver marks that was their due, while the crusaders received 50,000 silver marks. A further 100,000 silver marks were divided evenly up between the crusaders and Venetians. The remaining 500,000 silver marks were secretly kept back by many crusader knights. Speros Vryonis in Byzantium and Europe gives a vivid account of the sack:.
Output (English): What did they use as a base?