I wanted to pop open the lid on this discussion, as there seems to be little or confusing discussion around this topic. I personally do NOT currently know how to create deep shadows or very dark spaces on maps. I have a map concept that is being built on the Alpine base map. It requires me to create areas that have no light except for a candle or flashlight here and there. Not the whole map, mind you, because much of the action can also happen outside.
So I put it to you, the Forging Community, to help myself, and others, come up with some creative solutions!
Have at it!