The circles are invading and it's up to you to defeat them. Build up your multiplier by destroying your enemies. The higher your multiplier the higher your score will be. Multiplier points can also be used to purchase power-ups. Each power-up make you stronger in their own way.
Shields protect you from incoming enemies. they cannot be destroyed by enemy bullets.
The shooting power-up greatly increases your rate of fire and damage of your bullets.
The drones power-up calls in tow helper drones that help you shoot enemies. Drones are very useful for clearing the screen of enemies.