Its generally dark here, excepted for some lit areas indicated on the map. Marines can get in through a marine sized crawl space at each side. The Predator can not come in because of its size. And the marine can watch him wait out there. Although marines can use Predator tunnels as well.
Lower level is like a giant fork shape.
The out lanes are exposed to the level above and these lanes both have exits. The upper level is accessed through a predator tunnel as seen in the top left corner of the room. At the other end are two exits which are primarily Predator exits. Therefore marines may be injured if they exit here since it is on a high level of fall. Being on the upper level, you are able to see whats going on on the two outer lanes of the lower area.