Complex sentence: The French (both soldiers and pilgrims) having been taken by surprise, were trapped; those who tried to escape were caught and killed, and many men and horses and much of the baggage were cast into the canyon below.
Split into simpler sentences: The French (both soldiers and pilgrims), taken by surprise, were trapped; those who tried to escape were caught and killed. Many men, horses, and much of the baggage were cast into the canyon below.