Nethack is an extensible game, in the sense that you can add new dungeons and levels to any nethack installation. Both dungeons and levels are designed on description files (.des files) and then compiled into opaque data files.

The description language is a pretty and visual one: you paint the map in ascii, and then declare objects and monsters.

paxed from #emacs wrote a major mode for edit nethack description files. It is extremely useful for anyone hacking nethack files, and paxed claim he has no time to maintain the mode. So i am hosting the mode here.

MAZE: "Arc-goal", ' '
GEOMETRY:center,center
MAP

                                  ---------
                                  |..|.|..|
                       -----------|..S.S..|-----------
                       |.|........|+-|.|-+|........|.|
                       |.S........S..|.|..S........S.|
                       |.|........|..|.|..|........|.|
                    ------------------+------------------
                    |..|..........|.......|..........|..|
                    |..|..........+.......|..........S..|
                    |..S..........|.......+..........|..|
                    |..|..........|.......|..........|..|
                    ------------------+------------------
                       |.|........|..|.|..|........|.|
                       |.S........S..|.|..S........S.|
                       |.|........|+-|.|-+|........|.|
                       -----------|..S.S..|-----------
                                  |..|.|..|
                                  ---------

ENDMAP

Download the major mode: http://es.gnu.org/~jemarch/downloads/nethack-des-mode.el

Bug reports and improvement suggestions to jemarch@gnu.org. Thanks.