Hold the line against wave after wave of attacking enemy formations.

• Arrow keys or A/D (or the on-screen buttons on touch) to move left and right.
• Space, or the fire button, to shoot upward.
• Most enemies hold formation, but watch for clusters that break off and swoop down at you - they're faster and harder to predict.
• Avoid enemy fire and collisions. You have a limited number of lives, shown in the HUD - lose them all and it's game over.
• Clear a wave to advance to the next, tougher one.