The Doom floor is an entirely randomly generated floor accessed by exiting a Ran-geon level through the Doom or "trap" gate. Unlike the rest of the Ran-geon where monsters and traps are populated based on the current Monster Level, the level of each trap and monster on the floor is chosen randomly between the current Monster Level and level 9999, making the Doom floors extremely dangerous.
Doom floors tend to be much larger than other dungeon floors, and are usually populated with Immortals, Mimics, and boss-type monsters. All kinds of traps (incluing Hiyo traps) spawn, as do force fields and fire/ice jets. Item-bearing chests can be found, but the player should approach them with caution due to the risk of a chest being a high level Mimic. When possible, they should be tested first by hitting it with a Blessing, Curse, Haste or Slow trap.
The Doom floor is numbered the same number as the floor it was entered from, thus if the player enters the Doom floor from the 2nd floor of a Ran-geon, the player will exit the Doom floor to the 3rd floor of the Ran-geon. Doom floors can also act as Transition floors in Tri-geons, if the player exits through a Devil or Hell gate, they will end up in the Chaos branch, otherwise if they exit through an Angel gate, they will end up in Heaven.