Simple but challenging game! My highest score is about 30 seconds.
I have a couple of suggestions. For starters, I should be able to view my high score at the game over screen. Second, I think the enemies shouldn't start firing as soon as they spawn. Lastly, put a key / input pause in the game over screen so it doesn't end immediately. Most likely I am still pressing on my movement keys when the game over screen is shown so it ends immediately.
Keep up the good work, dude!