Hmmm... think of that time-travel movie where the surviving humans lived in bee-hive like places on the insides of large cliffs over the river, the surface was riddled with grimlock like beings who could disappear into the ground...
Also, for movies, think 'Congo', the lost city/king solomon's mines guarded by feral gorilla's (dire gorilla's anyone?)...
If you have the Nyambe sourcebook, pillage that for ideas as well...