The environment concept
The environment concept, as the name implies, is the concept design for your game's environment. The difference between level design and environment design is that the latter is more about the art—how to use artistic elements to make the scene look more interesting. Many important guidelines will be given to the concept artists before they start using the environment concept. Artists need to know the visual style, color mood, detailed description of the environment, and most importantly, the level layout.
Here is the environment concept I did for this book. I split the layout into several parts and illustrated each of them based on the visual style and color mood I picked earlier. Note how I use lighting to guide player toward the correct path.