Also semi-transparent polygons on top of players don't look very good. Have you considered making background scenery out of them?
I thought the transpalent poly was not a good choice but I still considered it would be dark in a cave. I know a map is not the mirror of reality somewhat. As it's written, this is a practice of PolyWorks to me. And it has been done almost so I will exchange the poly for background scenery, though I'm not liking a wallpapered map as you perhaps know.
About the exits from the cave, it will be a solve so that I make a roof or something "Only bullets collides" at the openings or another route to the end of map.