Complex sentence: The Royalists had fought ferociously to hold off the Parliamentarian forces but Charles knew his army was not up to another day's fighting.He knew that he was outnumbered, and with the loss of Speen, his forces were vulnerable to another attack the next morning.
Split into simpler sentences: The Royalists had held off the Parliamentarian forces but Charles knew his army was not up to another day's fighting. He was outnumbered and with the loss of Speen, his forces were vulnerable to another attack the next morning.