Complex sentence: The city, which was ringed by forts known as the National redoubt, was besieged to the south and east by German forces, which began a bombardment of the Belgian fortifications with heavy and super-heavy artillery on 28 September.
Split into simpler sentences: The city, which was ringed by forts known as the National redoubt, was besieged to the south and east by German forces. A German bombardment of the Belgian fortifications with heavy and super-heavy artillery on 28 September.