Has this been done?
 

Secret door to a ladder which enters to the attic.

http://atlas.imagemagician.com/images/r ... orview.jpg
http://atlas.imagemagician.com/images/r ... rview2.jpg
http://atlas.imagemagician.com/images/r ... rview3.jpg
http://atlas.imagemagician.com/images/r ... rddoor.jpg
http://atlas.imagemagician.com/images/r ... rddoor.jpg

I havent seen any other maps doing this...

I'm also going to have a rotating bookcase (without a hinge so it will just turn on its axis) to go into a secret room.

The purpose of the hidden places is basically an ammo store, and in the attic's case, a way to the roof as well, which leads to a few good sniping locations and a telephone wire leading to the roof of another building, etc.

Hidden doors are fun!

By the way, does anyone know of a way such that a person could walk right through a wall?

hidden doors have been done

e.g

last castle

ah. didnt know.

How do I do a non-solid wall? Oh, and is it possible to have an invisible wall as well (so people cant go to a certain area)

For an invisible wall, just use the caulk texture. It'll be invisible in game but solid.