The Gods Left It Behind

From publisher blurb: Archeologists have been studying the primitive natves on the jungle world of Eniran. These natives have carved cities out of the thick jungle there. According to the native legends a god like race lives in a castle in the clouds above them. At first the archeologists thought that this was just native superstition but recently they discovered that there is, indeed, a castle floating in the clouds. One of the archeologists offers to pay the travelers to accompany him on an expedition to this castle and find out who built it.