What I do for places that are supposed to be underground is the following:

1. Put the floor texture as background.

2. Draw the walls outlining the chambers.

3. Draw a box around the map as a whole near the Border.

4. Combine the Walls and the bordering box from Step 3 as a polygon with the MULTI POLY tool. This make a Solid object with a 'hole' where the chambers are supposed to be.

5. Apply glows, bevels etc.

The result of the steps above means you do not end up with an area like your map above. The map is GOOD, but with the Wall Shadows makes it appear as a structure that is standing above a flat plane as opposed to be something carved out of the surround dirt/rock.