Well, the start is a bit... harsh. Maybe, wait for the user to press a key to fly, before starting the game.
Nevetheless, mobs are glitchy and the SFX "on hit" is scratchy. And when you understand the game, it becomes really easy to play, and boring quickly. There is no challenge after you know how to fly.
Your art is not bad by the way ! :). But you need to polish your game.