Complex sentence: In fact, after a prolonged siege, the city fell by treachery as accomplices within opened a door for the Mahdist forces to enter by night, whereupon thousands were roused from their sleep only to be slaughtered.
Split into simpler sentences: In fact, after a prolonged siege, the city fell by treachery, accomplices within opening a door for the Mahdist forces to enter by night. Thousands were roused from their sleep only to be slaughtered.