Now they are all part of the same gang. If I attack one of them, they all will jump me instantly. They will kill everything but each other now. You can affect the environment in all sorts of ways when creating a map. One of the most important and visually impacting way to affect an environment is lighting.
Lighting is done in two parts, ambient lighting and placed lights.
Ambient lighting controls the lighting of the entire scene. You can control the ambient light levels for interior and exterior separately. So if you want it to be bright and sunny outside and slightly darker inside buildings you can do that. Ambient lighting cycles with time of day. I have set the colors to what I think looks good for morning, noon, and night colors, but you can change them to anything you want.