Played on Windows, on some gaming website around 2007-09.
So you were a green turret somewhere on the screen, you couldn't move, just shoot, with birds eye view shooting circle bullets to grey neutral turrets which would then turn slowly green after a certain amount of bullets had been adsorbed by it, so then you would have control of that turret.
All this why the red AI turrets also placed somewhere on the screen, are trying to gain control of the grey ones as well
The aim of the game was to make all red turrets green then it would just move onto the next level, the background was just black and I think some grey turrets would have specific classes, like sniper or fast shooter.
The turrets are awfully familiar to the tanks in tank.io.