In this task, you are given a question and a context passage. You have to answer the question based on the given passage.
Q: What caused the collapse of the Russian defenses?, Context: For months each side had been building forward rifle pits and defensive positions, which resulted in many skirmishes. Artillery fire aiming to gain superiority over the enemy guns.:450–462 September saw the final assault. On 5 September, another French bombardment (the sixth) was followed by an assault by the French Army on 8 September resulting in the capture of the Malakoff by the French, and following their failure to retake it, the collapse of the Russian defences. Meanwhile, the British captured the Great Redan, just south of the city of Sevastopol. The Russians retreated to the north, blowing up their magazines and the city fell on 9 September 1855 after a 337-day-long siege.:106
A:
failure to retake it