World of Tiers influenced DnD, D&D

Gary Gygax lists it in Appendix N of the 1979 Advanced Dungeons & Dragons Dungeon Master's Guide, his published list of the fiction that shaped the game. Philip José Farmer is listed for "The World of the Tiers series". Its pocket universes, made by godlike Lords and linked by gates, are one of the sources for the game's idea of separate planes reached through portals.