Cook came up with the idea that everything would revolve around factions, and that those factions would be ideas taken to the extreme. He also felt that Sigil came about because it was natural, because the planes needed a crossroads, and that the campaign needs a center which could be both a place for adventure and a place to hide, where characters could get to and from it quickly. Cook decided to adapt the "Manual of the Planes" because the older material made survival on the planes too difficult or complex; he ignored anything that complicated gameplay, which left the "descriptions of twisted and strange creations".