Oathbound: Wrack & Ruin (2003)

From the back cover: The bulk of the City of Penance is a madly stacked and sprawling ruin, only superficially explored by the relentless rafters and treasure-seekers of the surface world. Every home in the city has a hundred others lost beneath it, and every city street stands atop an incredible three-dimensional maze of corridors, alleyways, and crawlspaces--a maze where time has left nothing unchanged, eroding and distorting the framework of magic and even the laws of physics themselves. Within Wrack & Ruin you'll find: - 3 new player character races - 5 new prestige races - 2 new prestige classes - New magic items, monsters, and spells