Spinvaders

From Rebisoft:

This game is based on the classic Space Invaders game. You control a gun turret at the bottom of the screen, and must fight off wave after wave of alien invaders. Of course they can shoot back at you and kill you by touching you.

The game levels get progressively more difficult as you go along, with the aliens increasing in numbers and speed, and acquiring better, faster bullets.

Spinvaders features a point multiplier mechanism based on your shot accuracy. Each hit will increase the point multiplier (to a maximum of ten), and each bullet that goes off into space will decrease the multiplier.

When the multiplier reaches ten, you acquire the "super-gun", which fires more quickly, is slightly larger, and is more devastating when it hits; The alien which is hit throws off a couple bits of shrapnel which are deadly to other aliens, and then it falls burning to the bottom of the screen, where it is deadly to you.