I don't know if this is a graphic error or a code error or if it's completely intentional but just not explained, but I get a game over when none of the creatures has reached the bottom of the screen. The creatures occasionally shift to one side, but it doesn't appear to be deliberate like in Space Invaders, and they don't all do it.
Is there a limited number of missiles that can be fired? If so, that should be indicated on-screen. I noticed that you lose points if your missile doesn't hit anything, I assume that's in place to deter spamming (which is why I ask about limited numbers).
In playing, I hit each visible creature with missiles, so I don't know if maybe the visible sprite vanishes but the game thinks the creature is still there, gradually moving down? I dunno. It seems to take the same amount of time for a Game Over whether I shoot them all or just let them all come down.
If the game is meant to be timed, show a timer. If the game is meant to be "survive as long as you can before you run out of missiles" then show the remaining missiles. If the only limitation is your ability to hit the creatures, then there's a major bug that needs to be fixed. Maybe once you've polished it up a bit more, you could actually have all 3 of those modes to choose from: Survival, Timed, and Limited Ammo.
It's a good start, but even aside from the bug it feels very easy. Two things I would definitely change:
-Zoom way out. Make the creatures and the missiles take up less space on the screen, so it takes longer for the missile to get to the top.
-Second, but very closely-connected: Only so many missiles allowed on-screen at once. Think about a game like Mega Man where you could fire as fast as you wanted but you could only have 3 bullets going at once, and you had to wait for the first bullet to either hit the target or go off-screen before you could fire again. This adds a strategic element of prioritizing targets but also makes it more important to HIT your target, because if you miss, you're penalized by waiting longer, even if it feels like a very short period, every split second makes a difference once you're playing at a really hard level.
I would also recommend using all original art assets. The Space Invaders are iconic, but you should be adding your own flavor to everything. It would be nice if they were animated in some way as well (but that's getting nit-picky). Maybe a nice looping soundtrack that isn't too loud or obnoxious, as well.