Underneath Phaeselis are many psionic foes as well as regular foes to do battle with and to fight. Brain moles, intellect devourers, and other horrors exist below the city. This allows you to go down below the city, encountering these monsters, traps, and other terrible things happening in dungeons.
Hellenistic culture plays a part in this setting, providing the backdrop, it isn't the focus. It's the Psionics rules in Ultimate Psionics and Psionics Unleashed that provide that focus as well as those of the PATHFINDER core rules. So while Phaeselis is set against a well researched background, many notions you see in a realistic world in other campaigns (most adults don't believe in monsters, everyone finds psionics bewildering, you never see lizardmen walking down the street) are not present here.
The Phaeselis Campaign is one which would label as high fantasy, like World of Warcraft, but psionics plays a very big part in the campaign. And the Phaeselis has many elements that one would label as horror and dark. But Phaeselis is about surviving in a world and reaching the apex of human potential rather than living on the low. It's clearly designed as an urban fantasy, as all the adventures take place within or below the city. It's a place with vast secrets waiting to be discovered.