Complex sentence: As a significant city of the Roman Empire it had trading contacts across the Roman world: in the Septimius Severus years Caesarea grew to be a very rich city with nearly 100,000 inhabitants, according to historian Gsell Leveau, Philippe.
Split into simpler sentences: As a significant city of the Roman Empire it had trading contacts across the Roman world. Under Septimius Severus it grew to be a very rich city with nearly 100,000 inhabitants, according to historian Gsell Leveau, Philippe.