The behaviour of light-sources and the shadow drawn by objects is normally not that important at all. If you are working on outdoor areas, the light falls directly from the top, so who cares? Well, "Detail-Addicts" do :) If you have somewhere a light-source and an object, which could prevent the light from getting to every little corner of that place, take advantage of it! The better the game between shadow and light is worked out, the more realistic and detailed the final result will look!
Shot one demonstrates the behaviour of crate- and building-shadows in the mainpart of the city in "The City of The Damned". The interesting thing about this is, if you take a close look on the skybox, you will nowhere find a light-source. However playing the map itself, you have always the feeling as the sun is rising/setting in the northern part of the city, and that's just because of the shadows I placed beyond big objects. But there is yet another useful optical carmouflage: With shadows there comes contrast and with contrast there comes eye-candy - believe it or not - but it works :) The outside area of my city isn't that overdetailed at all but with the frequent change of bright and dark areas, it looks like a heavy detailed part! (buildtime is uncountable, shadow for crates took about 2-3 minutes, for buildings about 10 minutes)
The second shows some of the big trickeries in the world of map design. What do mappers, if the are even too lazy to copy & paste some stuff from old parts of the map into the new one? Well, they simply darken some parts of the map and leave non-detailed parts lost in the darkness, so no one will ever see how lazy they have been ;) The room before the grey skullkey has been darkened exactly because of that reason but no-one ever cared and - thx to the contrasts - it still looked like I have put a lot of effort into this :)
Summary: Like in the RealLife™, you can create many different tricks with the strange behaviour of light and shadows, you just have to use it in your maps ;)
Extra-Buildtime: "uncountable minutes" (depends on level-size)