Windows:
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What is a window?

A window is basically just a hole in a wall that you can look through into
other areas.  Windows can have no thickness, thus being just a single line,
however they generally have thickness.  Thus, your windows will usually
have window sides, a window floor and window ceiling, so to speak.  What
this means is that the actual window area will be a seperate sector, with
a higher floor height than the room, and a lower ceiling height.  The
window sides will get their texture from the sidedefs on single sided lines.
2-sided lines (the front and back of the window) will have no middle
texture, allowing you to see through the window.  The window floor will get
it's texture from the window sector's floor texture, and the ceiling
likewise.

Since a window is just a 'hole' in the wall, you will probably want the
upper and lower textures on the window wall to line up with the walls on
either side of the window, so it looks like one continuous wall with a hole
in it.  This is done the easiest by unpegging the lower and upper textures.
When you create a window, you will want to look through it.  So, you must
make the middle texture transparent.  The upper and lower textures should
match the texture used on the ajoining walls, so it appears to be one
continuous wall.  You must remember to