How did Mahdist forces capture Khartoum in January 1885?

On January 26, 1885, Mahdist forces captured Khartoum by taking advantage of a gap in the city's defensive ramparts along the White Nile.1

Approximately 50,000 Mahdist troops stormed the city, bursting through the Masallamiyyah Gate to overwhelm the exhausted defenders. The garrison, which had been reduced to roughly 7,000 men, was massacred, and the British general Charles George Gordon was killed during the final assault. This victory marked the culmination of a siege that had begun on March 13, 1884, and it effectively extinguished Anglo-Egyptian power in the Sudan.213