An old man named Robert Wolff, looking at buying a house, hears a horn coming from an empty closet he had just inspected. He opens it and finds a portal to another world. A man inside throws him a strange horn before the portal closes. The old man hides the horn, then sneaks back in at night to get it. He opens up the portal and ends up in a bizarre place - a flat world with five tiers, stacked like a step pyramid, with mountainous cliffs connecting them.
He finds out that this world was created by one of the Lords (Thoans), a group of beings with fantastic technologies that keep them immortal. The lowest tier is populated by Greek-descended humans and monsters; Wolff falls in love with a nymph named Chryseis. When she is abducted by the Lord of this world, he must climb up to the top to rescue her.
And that's just the first book. Later books take on the Lords' worst enemy, the Black Bellers, and travel to multiple other universes created by other Lords.
There are six books in the series, plus one related book:
- The Maker of Universes
- The Gates of Creation
- A Private Cosmos
- Behind the Walls of Terra
- The Lavalite World
- More Than Fire
- Red Orc's Rage (which uses the World of Tiers as a fictional element within the story but doesn't involve the main characters from the other books).
This series provides examples of:
- A.I. Is a Crapshoot: Black Bellers — sentient minds that "grew up" in storage devices for consciousness transplantation.
- Ancient Artifact: Effectively, all old technological devices, since surviving Thoans usually can't make more. The Horn of Shambarimen is particularly notable, being unique and extremely useful — it can open a portal in place of any inactive teleportation device.
- Amnesiac God: As revealed at the end of the first book, Robert Wolff is Lord Jadawin, but lost his memories after being defeated by another lord and being stranded on Earth.
- Changing of the Guard: The protagonist shifts in the third book, going from Robert Wolff to Kickaha.
- Dimensional Traveler: Paul Janus Finnegan (AKA Kickaha the Trickster) and Robert Wolff spend much of the novels traveling through artificially created universes.
- Grand Theft Me: Black Bellers are capable of taking over the bodies of human hosts.
- I'm Cold... So Cold...: Last words of a Drachelander (German-descended) knight in the first book, after he was swarmed by enemies and the protagonist arrived just too late to save him: "'siz kalt."
- Interdimensional Travel Device: People can travel between the artificial universes of the setting by using gates. Gates can be activated by various means, including tokens and playing music on a special horn.
- Healing Factor: All Thoans and some dwellers of the pocket universes able to regenerate lost and damaged body parts.
- Monster in the Moat: A castle in the Dracheland tier is described as having a river dragon living in its moat.
- Portal Cut: One of the many interdimensional teleport gates is a trap that dismembers by severing the connection and the intruder.
- Portal Network: Portals can be used to connect between worlds. A common portal design is two crescent moons, touching at the tips.
- Recursive Precursors: In Behind the Walls of Terra, the Thoan home universe itself is stated to have been created by another, unknown race.
- Red Pill, Blue Pill: An aging man with a failed life and a shrewish wife is in the basement of a tract house he is buying when a door between the worlds opens up. He can stay with his living-death retirement or leap into the utter unknown. Soul-killing safety vs death-or-glory.
- Significant Monogram: Paul Janus Finnegan's initials are the same as Phillip José Farmer's. This is something of a recurring easter egg in Farmer's works.
- Swap Teleportation: Swapping teleporters co-exist with portals, and the swapping is explained early on. Usually, the swapped volume is a cylinder wide and high enough to fit a single human, or several.
- Villainous Incest: Incest is perfectly acceptable among the Lords and Ladies of the pocket.
- What Happened to the Mouse?: The eventual fate of Robert Wolff/Lord Jadawin and Chryseis is never revealed after their disappearance in A Private Cosmos.
- World Shapes: The World of Tiers takes the form of a world-sized Tower of Babel, just that the continent-sized layers (tiers) are way wider than they are tall. And the top layer has the Lord's palace on it.