I have one more thing to add to evgo, as I agree with him, my finger gets pretty tired. I find it hard to aim, I have yet to hit anything on purpose. I have one more issue, when you go on the top or bottom of the screen your aiming goes down towards the center, making it very difficult to hit something on the top on the bottom of the screen. Also, the ship moves off the side of the screen when you go left, or right. One more thing, the sound before someone talks is annoying, please change that.
I am happy to see others using Unity 3D to create games too.
It is pretty close, one thing you forgot, when the aliens number dwindles, there speed increases. It is way too easy now. Also, if you to add an extra touch, animate the march, if you notice in the original, they each move one frame apart in timing.
 "Replicator"){kind=icon}
 "Space Arcade (the game!)"){kind=icon}
 "Space Invaders"){kind=icon}