Complex sentence: At this time the French army under Richemont arrived from the south, having crossed the Aure and now approaching the English force from behind, they numbered almost 1,200 men-almost all mounted due to the pace of their march.
Split into simpler sentences: At this time the French army under Richemont arrived from the south, having crossed the Aure and now approaching the English force from behind. They numbered almost 1,200 men-almost all mounted judging from the pace of their march.