In this directory, you will find three directories called
Info, Single, and Multiple.

Info contains Doom II official Frequenty Asked Questions
and a Secrets text file to help you play the game.

Single contains Doom II add-on levels that will work
for one-person game play.  You will want to open the
text file that comes with each .WAD level to determine
which Map it replaces.  As an example, suppose that
the text file KILLER.TXT for a level called KILLER.WAD
says it replaces Map 02; then at the
C:\DOOM2> prompt you type DOOM2 -FILE KILLER.WAD -WARP 2
to play that level. 

Multiple contains levels that support multiple-player
play; note that many of these are also found in the
Single directory.  You will want to open the
text file that comes with each .WAD level to determine
which Map it replaces.  As an example, suppose that
the text file KILLER.TXT for a level called KILLER.WAD
says it replaces Map 02; then at the
C:\DOOM2> prompt you type DOOM2 -FILE KILLER.WAD -WARP 2
to play that level.  You will also want to check whether
the WAD supports Cooperative or Deathmatch or both types
of play.