Well the title got me pulled in, and the fact that it was space invaders and you admit that was cool, it was a fun game to play.  Not innovative for obvious reasons, but fun to play for sure.  Though I am a bit confused as to how it fits the theme I could still sink some time into it.  Plus, Godots the best :3.  

Anyway, some finer points:

• The instant spawning of the next level of enemies added to the difficulty a bit more than I think it should, just cause depending where you were at as you killed the last wave could impact your ability, or ease, in killing the new wave.  Plus it means there's essentially no down time.  Unless of course, you uh... take advantage of the walls.
• As for the walls, I just figured I'd mention it, it's... sorta an interesting mechanic.  The fact you can phase out of the map can be kinda useful.  Adding some sort of exit... or city etc. that can be damaged while you hide could make it a useful thing(I'm assuming bug) but also something you have to balance out.  
• This is more preference, but I think changing the fire from a "just_pressed" to a "pressed" could help.  That way you could get a nice stream of bullets relatively easily.  Especially since currently you still can, but it's just harder due to it relying on whether the button was just pressed.

Overall though even with it being a clone it's an interesting game to just mess around with.  Especially just to see another Godot one :3

Great art and gameplay! Good job!

Please check my game as well ;)