Complex sentence: The 17th August saw the fith bombardment of Sebastopol with 600 Allied guns, which lasted for four days, and the sixth and final bombardment, which began on 5th September with 775 Allied guns and lasted for 3 days.
Split into simpler sentences: The 17th August saw the fifth bombardment of Sebastopol with 600 Allied guns, which lasted for four days. The sixth and final bombardment, which began on 5th September with 775 Allied guns, lasted for 3 days.