A floating building shaped like a double cone, the bottom half inverted. It was a short, wide, ornate building, with lights on the top half, in the Civic Center of Hotrufan City, a City Builder city on the Ringworld. [Spoiler alert: Ringworld] The home of Halrlroprillalar Hotrufan (aka Prill), it was encountered by the First Ringworld Expedition in 2851. A speed trap drew vehicles violating local air traffic speed limits into a prison in the lower half of the building. The prison consisted of a conical pit with concentric rings of cells around the walls, and an airborne "Sargasso Sea" of rusting vehicles occupying the center. The expedition's flycycles were captured and pulled into the prison. Eventually Prill was persuaded to let the expedition turn the building into an ungainly vehicle, the Improbable, q.v.