The Dungeon is chiefly a deep, dank dwelling of monsters to overpower and
treasures to be gained.  You gain more experience points from defeating a
monster in the deeper levels, simply because they generally get more powerful
as you descend.  Each level has stationary monsters, wandering monsters, a
magical teleport, trapdoors, thieves, and a cleric.  You can only obtain a map
of the level you are on from defeating a monster that possesses one.  Once you
leave that level, the map is totally useless and you automatically discard it.
There are a few other surprises that await your discovering them.  Good luck!