Would you like to match your favourite character against a dinosaur? Or does exploring a primeval jungle appeal?
Whether you decide to base an entire game here in Karkus, or merely have characters chance upon it (perhaps through a dimensional rift or as a remote 'Shangri-La' style location) it is ripe with possibilities for adventure.
Giant plants, dinosaurs, sabre-tooth tigers and more abound. The environment alone provides plenty of challenge and environmental disasters can add to it. Perhaps the characters can train a riding raptor to speed across the grasslands... But there are folks already living here too with whom they can interact. (Or of course, amongst whom they number if your whole game is set here.)
Everything is laid out in sequence, beginning with a brief gazetteer. Notes on technology (surprisingly advanced at times) and religion follow, and then we are introduced to the natives themselves. A diverse lot too, with some interesting and unusual features... like the long-necked Corsath who display some of the features of a brachiosaur, and are nomadic farmers and warriors, rather good with a longbow and seemingly capricious - your best friend one day, a deadly enemy the next. Others look a bit like bipedal crocodiles and so on. Humans live here as well, but are not native. All the relevant game mechanics to play any of these races are provided (apart from Humans, use the ones from the Pathfinder core rulebook!).
Next comes information on the native flora and fauna. The plants are fascinating, the animals are decidedly 'Lost World' in nature, dinosaurs and other strange beasties. Then comes some new feats appropriate to the new races and the way in which they interact with their environment.
It sounds a fascinating place to visit... as a hidden land or something accessed via a portal it could prove an exciting addition to your campaign world.