Det finns inte massor av information - från länk :
City Builder colony worlds
Ten planets in stellar systems in the vicinity of the Ringworld, reportedly colonized by the City Builders. [Spoiler alert: Ringworld] The Map Room in Heaven displayed ten globes; Louis Wu thought these were the colony worlds. According to the story told by Halrloprillalar Hotrufan (Prill) to the First Ringworld Expedition, the City Builder race originated on one of these planets, five of which were visited by her ramscoop ship, the Pioneer, on its regular trade run. Prill claimed two of these worlds were densely populated before the Ringworld was built, but had afterward been abandoned, the populace moved en masse to the Ringworld. Nessus speculated these worlds included some in Human Space, and Louis Wu even briefly speculated Earth itself was once a City Builder colony [2]. [Spoiler alert: The Ringworld Engineers] However, in light of the discoveries of the Second Ringworld Expedition, it appears this was one of Prill’s "tall tales" aggrandizing her people, and that the City Builders originated on the Ringworld. No other account mentions these colony worlds; therefore their existence, abandoned or not, is unconfirmed.
Reference: Ringworld chs. 16, 21
Jag tror att det var lite insikt om detta i Ringworld RPG. Önskar jag hade en kopia bara för att läsa.