Really just make it dark like how places normally get dark - make sure light can't get to it. The compilation tools (specifically the RAD step) do some clever maths to work out how much light is where on the map. So if you put a wall in the way then there won't be any light (as far as I know doors don't block light but I could be wrong).
One thing I found out will make you, the player, very dark is to set the patch of ground you want to be dark as a func_wall. When you wall over it, you will turn black. I am going to use this to make a little corner super-ultra dark. Try it! Experiment by spawning a hostie on it, and they will be completely black.
Haha havokk .... but Down Rodeo.... i fot an stair down underground... and outside its sun, so how can i block the light from going down in the long stairs? and you should be able to walk trough the wall if there shall be any wall , thanks.
Well the problem is nubbcake it's based on semi-realism, where light could go in real life it can go in CS. Almost. To a certain approximation anyway.
M d'V: Adding a zero strength light is like a lamp in a room that isn't turned on: it adds no light. I don't think CS (or any other software for that matter) has a concept of a negative light.