Defend the planet from an endless procession of alien waves. Every new wave is freshly generated and moves twice as fast as the one before — shelter behind the bunkers, pick off the bonus UFO for extra points, and see how far the doubling can take you.
This follows the original 1978 formula closely: a grid of aliens marches side to side and steps down every time it hits the screen edge, while three destructible bunkers are the only cover between them and your cannon. A rare bonus UFO crosses the top of the screen for a handful of extra points if you can snipe it.
Waves are generated endlessly rather than stopping at a fixed final level, and each new wave's aliens move at double the horizontal speed of the last — the grid also thins out and speeds up further as you clear it, exactly like the arcade original. Three lives per run; if the aliens ever reach the bunker line, it's game over regardless of lives remaining.
Sound effects and the classic accelerating four-note bassline are synthesized live with the Web Audio API rather than played from audio files, and your best score is remembered on this device between visits.
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