Complex sentence: Caesarea would become the capital of the Roman client kingdom of Mauretania, which became one of the important client kingdoms in the Roman Empire, and their dynasty was among the most loyal client Roman vassal rulers.
Split into simpler sentences: Caesarea would become the capital of the Roman client kingdom of Mauretania. The kingdom of Mauretania became one of the important client kingdoms in the Roman Empire, and their dynasty was among the most loyal client Roman vassal rulers.