Complex sentence: Workers piled up at the entrance of the stairway because the stairway was too dark to see their way down the steps, that had no landing and were crushed by the door because of the mass panic and fear from the people running to the dark stairway.
Split into simpler sentences: Workers piled up at the entrance of the stairway because the stairway (which had no landing) was too dark for one to see his or her way down the steps. Many were crushed by the door because of the mass panic and fear from the people running to the dark stairway.