Dungeons of Doom
The Dungeons of Doom is the name given to the first half of the Mazes of Menace. They are 25 to 29 levels deep. It is here that most players meet their deaths, typically due to YASD. It also contains all stores, and the vast majority of optional branches, including Sokoban, the Gnomish Mines, the Oracle, and Medusa. Consult the diagram in the article on the Mazes of Menace for a schematic showing the locations of branches and special levels.
The player starts on the first floor of the Dungeons of Doom and has to work their way down, eventually passing below the Castle into Gehennom. After retrieving the Amulet of Yendor from Gehennom, they must then return to the first floor, and climb the stairs to enter the Planes.