Apocalyptic

Humans thrived in peace but one day, the demons from the underworld invade earth to conquer it. The war between humans and demons lasted 20 years but humans faced defeat with 40% of humans survived the war. 50 years have passed since the war and the humans are slowly going extinct, the demons build garrison around the earth to control and monitor humans and prevent them from rebelling against demon,the demons also build fortress in each region to prevent the humans from escaping, but there's 4 fortress different from a normal fortress, but no one know what they do or what is their purpose