Accessing maps 16 and 17 in Sega 32X Doom:

To access either map 16 or map 17, you must start the game
from map 1, and not use any cheat codes at any time.  When
played this way, map 16 can be accessed from the exit (the
only exit) on map 15.  Map 17 can be accessed from the
secret exit on map 3.  (For details on finding the secret
exit, click here.)
If the game was started on any map other than map 1, or if
cheat codes are used during the game, then the secret exit
on map 3 will lead to map 4, and the exit on map 15 will
end the game.

Also, a player must start on map level 1 and play the game
all the way through without using cheat codes in order to
see the entire ending sequence.  A game won any other way
will result in the credits being shown, and then the game
simply halts, displaying a plain textured screen and a "DOS
prompt".