Philip Jose Farmer's The Dungeon Books

The Hidden City: Nearing the zenith of their quest, Clive and Neville Folliot and their comrades from time and space are scattered through a ruined city. Beneath the decaying streets waits the greatest danger of all: the Ren and the Chaffri, inscrutable masters of the Dungeon! The Final Battle: Fleeing from the mysterious creators of the Dungeon, Clive breaks through to the ninth level. Stranded in a freezing polar wilderness, he struggles to find his missing comrades and to triumph at last against the murderous masters of the Dungeon!

Finding himself on a vast prison planet on which his twin brother is trapped, Clive Foliott encounters an array of adversarial dwarves, cyborgs, and aliens from different parts of time and space. Original.