Very cool
Sound is out of sync; there is a delay between firing/reloading and the sound effect.
BTW, you could really enhance the game with more sound effects, dialog, or music. Sound is a lot more important than most people realize
Multiple difficulty levels affecting the time betwen an enemy's entrance and attack might be a good idea. Some people want more challenge, or just have faster or slower reflexes than the norm.
Cool game! Thanks!