Complex sentence: The destruction of all three staircases in the North Tower above the impact zone, when Flight 11 hit, made it impossible for anyone above the impact zone to escape, making one hundred and seven people below the point of impact died as well.
Split into simpler sentences: The destruction of all three staircases in the North Tower above the impact zone, when Flight 11 hit, made it impossible for anyone above the impact zone to escape. One hundred and seven people below the point of impact died as well.