It looks good and plays relatively well. The upgrade menu needs some love, at first I didn't realize I could scroll down to upgrade my other stats. Enemies should not spawn in the middle of the arena (which leads to them spawning on you and exploding right away): I would spawn them outside of it and lead them toward the player, or check if the player is in a specific radius before spawning.
Another easy way to improve game feel I think would be to increase the base fire rate of the player, but increase the base HP of the enemies. As it is, it is pretty frustrating to miss your only shot, and not very fun to wait for the next one to come up.