The idea to spawn enemies in the dark areas is cool. You can also add spawning in the shadows or behind the buildings.
I think you need to put somewhere that you can shoot with LMB. It is not stated anywhere and not obvious. Also you can shoot only in the dark area, where it is hard to see enemies. I'm not sure if health is regenerated, but it is really hard to catch up if you ever want to look back to figure out if there are still enemies behind.