Complex sentence: They were forced to withdraw by French resistance, but returned the following day with reinforcements, and heavy fighting took place within the fortress walls for another two days until a French bayonet charge forced Zayd's fighters behind the fortress's perimeter.
Split into simpler sentences: They were forced to withdraw by French resistance, but returned during the day with reinforcements. Heavy fighting ensued within the fortress walls for three days until a French bayonet charge forced the rebels back behind the fortress's perimeter.