Complex sentence: They were again forced to try to defend themselves on a hill, the muddy field conditions and narrow nature of the area made it difficult for them to utilize their horses, so the force mostly dismounted and fought on foot.
Split into simpler sentences: They were again forced to try to defend themselves on a hill. The muddy field conditions and narrow nature of the area made it difficult for them to utilize their horses, so the force mostly dismounted and fought on foot.