True, I didn't expanded much from the "shoot to kill the enemies!" lie. It took most of my time to get the gameplay working decently and to create the enemies.
The enemies do produce a sound when they spawn, and that should make the player be aware that enemies could appear from behind (as LilNarwhal suggested, the enemies could make a sound while moving, which would make it easier to be aware of your surroundings).
Thanks for the rating and comment!