its being creative and trying to make a small space look big
Say u have a cliff, try to have that go up to the height where the player at any point will not go past (height wise). Make the sky from that point the roof, that way at certain points u dont need sides to be sky! now u have incorporated the terrain into the out doors. Now u can make
a couple cracks in the cliff, put a couple brushes directly on other side of wall seeling the cracks. Make them sky brushes so it gives a neat effect, u can even do this in corridors if u like. This of course is just idea to get u started. Sky doesnt have to be outside map it can be part of it, no matter where u put sky it will still look like sky if that makes sense. SAo it doesnt have to be an outlying source but a part of terrian at parts. (Note it also works as clip) so u dont have to put clip brush infront if u dont want peeps to fall or walk past that point.(hint hint)
Enjoy taking this to the next level theres alot u can do with it so have fun
cheers [DDHLP]copper