Sides, not size, are everything. The number of sides a polygon has is its lives and its power.
You are the blue turret at the bottom. Aim with the mouse or your finger and hold to fire green projectiles; let go to stop.
Red polygons fall toward you. An n-gon that reaches you costs you n−2 sides. Each enemy you kill fills your white XP bars.
Shoot the green shape (left) when your white bar is longer than the green bar: spend the XP, grow the green bar, and add a side to your projectiles (more damage).
Shoot the cyan shape (right) when your white bar is longer than the cyan bar: spend the XP, grow the cyan bar, and fire faster (an n-gon cyan shape = n shots/second).
A bigger projectile pierces smaller enemies, carrying on with the sides it has left.
Win when the green and cyan bars touch. Lose if your blue shape drops below 3 sides.