Very nice game!
I find the invincibility power-up a little pointless though because I think getting to it is probably harder than the 5 seconds after. Also, the bullets sometimes deflect off my shield and into myself, which might be a bug that is worth fixing (maybe once a bullet bounces off the shield, it simply cannot do damage). The scoreboard also didn't seem to add my score to it, but I'm sure that's because it needs to be manually updated.
Overall, a great game! It has some nice music too, 5/5!