Secret areas:

As you should know by now, when you complete an episode, a 'total secrets
discovered' tally is calculated, based on how many secret areas were
discovered during that episode's play.  So, how is a secret area defined?
Well, each and every secret area is a sector with a 'secret area' sector
type.  If the player walks into this sector (by crossing one of the sector's
lines), that secret sector is marked as discovered.  You may be aware of
a line attribute that is called 'secret'.  This doesn't count into the
secret areas total at all, and it's function will be discussed shortly.
Only secret sectors will count toward the total at the end of the episode.
So, when creating secret areas, you the map designer need to think about
what should be a secret area, and why.  At what point should your secret
area be considered discovered?  The only way to mark an area as discovered
is to have the player walk into a secret sector, so you need to decide where
you want to place this secret sector exactly.  It should fall from wall to
wall normally, so that a player will always discover the area, and can't
walk around the edge of it or anything.  Based on id's original maps, and
the PWADs I have seen, there are several catagories of secret areas, and I
will discuss these all now.  Keep in mind that this isn't the total,
absolute list or anything.  The best secret areas are usually ones that are
very creative and/or a unique type of secret that no one has ever done
before.

Secret areas behind a hidden door:

This is probably the most common of all types of secret areas.  The player
walks up to a wall and presses the spacebar to open a door that looks like
the rest of the wall, except the wall has a slight different from the rest
of the walls around it.  Common ways to make the secret door detectable to
the observant is by using a different type of wall, or by shifting the wall
vertically or horizontally a little, or by illuminating the wall, or maybe
having a pattern of powerups in the shape of an arrow pointing at the door,
or having the automap show the door, but not in the normal view.  This last
method is usually the worst, though.  Normally you'll want to hide the door
on the automap (make it look like just another wall).  This is what the
secret attribute of a line is for.  It makes the line the same color as a
wall, so the player can't tell it is a door.  If you want to, however, you
can leave no clue whatsoever that there is a secret door there, and the
player will simply have to try pushing on walls to find them.  Some people
think you shouldn't do this, other think it is ok.  You are the author of
your map, however, and I beleive that you should have full control of it.
So, do whatever you want to do.  In designing maps, think about what you
like and dislike when you play, and try to make your map according to that.
With secret door,