Eos is a planet, we have two major continents. Before the darkness came there were two ruling forces - The Niflheim Empire and the Kingdom of Lucis. We Niflheim had controlled most of Eos outside the Crown City of Insomnia prior to the world going dark. Gralea was the capital of the Empire and one of the first cities to be overrun by daemons as the Emperor was using the Scourge in experiments which ended badly. I've been a handful of times since but no one spends more than a few days there now.
Most people have moved to Lestallum, the power plant there was able to keep running and provided a safe haven. But there's a lot of us that still travel to other outposts and cities to fight, to find resources, to help those who chose to stay.