The Forgotten Ruins are a secret hidden area below the The House of Blossoms inside the Underground Ruins. It was once part of the Keeper Compound in ancient times, it is now long abandoned, crumbling apart. The City keeps its own secrets. The House of Blossoms itself was also part of the compound. Inside the ruins is the Scriptorium or the Keeper Library.
- Several areas in the Forgotten Ruins including the central Library section resemble the Keeper Library as seen in Thief: Deadly Shadows, though crumbled after centuries of disuse.
- The Glyphs used to enter the library are same glyphs seen in previous games.