Your map is total bright due to a leak, or a lack of light entities. When you have no light entities, the map is always made full-bright. Adding a light would be a good thing to do.
As for the layout of your map, it's way too open. You should start with creating one closed room and connecting it with a hallway to another room, and so on. Then when you want to see the sky behind a window or trough an open ceiling, you should add a skybrush behind this window or over this ceiling. The whole box isn't necessary, very bad in fact due to it's many sideeffects. Just be sure to have sealed your level from the outside, the void.