Rereading this a few years ago, it struck me that Halrloprillala's people didn't seem advanced enough to be convincing as builders of the Ringworld. Now I wonder if Niven had plans for a sequel from the start, or it's just me back-projecting as a result of my knowledge of the sequel? (And then there's the possibility that Niven just couldn't manage to convincingly write a civilization both advanced enough to build a Ringworld and incompetent enough to collapse as thoroughly as the plot required.)