Am I really gonna be the one to say it? This is NOT a tower defense game. Tower defense involves building units (i.e. turrets, towers, traps, etc) that defend against incoming enemies, often along pre-set paths. While this is an intriguing little game, and does have potential for improvement, I have to point out that it is misclassified. As has been said, I would look into adding power-ups/upgrades to give the player more of a fighting chance, perhaps even multiple weapons you could switch between. I would also think about a larger enemy variety, and perhaps even boss levels. Great attempt. Keep at it.