Attached to the center of a circle, kill the polygons before they reach you. As your health shrinks, so does the playing field.
Use cash to purchase upgrades (spread shot, strong shot, explosive shot, etc) or repair.
Enemies which increase in difficulty over four levels, each with a uniquely crafted boss should bring a challenge.
My first Flash game, so comments much appreciated.
Edit: Sorry, seems I used the wrong version when I uploaded. Sorry for the inconvenience of not being able to finish, and for the horrid newgrounds stage colour as a background. It will work fine as soon as the update goes through.
Also, as your upgrades reduce the points you recieve, you are largely in control of the game's difficulty. Also, keep in mind that there IS a hard mode (which will certainly frustrate you). Normal is scaled back about 10% from needing to play perfectly assuming minimal upgrades.