Complex sentence: This allowed Griffith to bring most of humanity within his city walls due to the monsters from other planes now roaming the world with Apostles now serving mankind's protectors while allowed to satisfy their bloodlust in the fighting arenas within the nearby fortress of Pandemonium.
Split into simpler sentences: This allowed Griffith to bring most of humanity within his city walls as various monsters posed an increasing danger to the outside world. Apostles under his command are now serving as mankind's protectors, and some of them satisfy their thirst for battle in the arenas within the nearby fortress of Pandemonium.