This is the highest level of the Dungeons of Doom, and so located on DL 1.
The top level (as for all levels above DL 4) is ineligible to leave bones files.
This is an ordinary Dungeons room-and-corridor level in almost all respects. It is the level on which the player character begins the game (at the upstair); going up from the level will mean escaping the dungeon (and ending the game) unless you are carrying the Amulet of Yendor, in which case you instead arrive at the Plane of Earth. Cursed potions of gain level can only be used if you are carrying the Amulet.