Bit Defense

About Bit Defense

Bit Defense is a wave-based fixed-screen shooter. You control a small ship at the bottom of the screen. Aliens arranged in a grid descend slowly from the top, weaving back and forth as they move. Your ship can shoot upward and move left and right. Hit aliens with your shots to destroy them. Aliens periodically drop their own shots downward toward you. Get hit and the ship is destroyed. Clear the screen of aliens to advance.

The format was created by Tomohiro Nishikado at Taito in 1978. The original arcade game was a global cultural phenomenon and remains one of the most influential video games ever made. Many basic conventions of action games, including the player ship at the bottom and enemy ships descending in formation, originated with this format.

Bit Defense is Susam Pal's MIT-licensed implementation of the format, included on this site with attribution and a link to the source. The whole game is one HTML file, around 45KB, with no external dependencies. The pixel-art aesthetic mimics the original arcade machine's chunky low-resolution display.

The grid of aliens moves in unison. The whole formation slides one tile sideways at a time, then shifts down a row when it reaches the screen edge. As you destroy aliens, the remaining ones speed up. So a half-cleared screen has fewer threats but each one is moving faster. The acceleration is part of the difficulty curve.

Controls are arrow keys for movement and a single key, usually space, to shoot. Touch users get on-screen direction buttons and a fire button. Each shot is a single bullet. There is a small cooldown between shots. You cannot just hold the fire button; you have to tap rhythmically.

Strategy starts with positioning. The ship can move freely along the bottom of the screen. The aliens descend in patterns that are easier to hit from certain horizontal positions than others. Most players settle into a habitual movement pattern: stay near the center, move out to one edge to shoot a column, retreat back to center.

Targeting priority is the next skill. Some aliens are worth more points than others. The smaller aliens at the top of the formation are harder to hit but worth more. The larger aliens at the bottom are easier targets but lower scoring. The optimal order depends on your reflexes. Beginners should clear the easy targets first. Stronger players go after the high-scoring ones early when there is less screen pressure.

Defending against alien fire is mostly about staying mobile. Aliens drop shots at semi-random intervals. The shots are slow enough to dodge if you see them coming. The trick is to keep moving even when you are not shooting at anything. A stationary ship is a much easier target than a moving one. Even small lateral movements throw off the descending shots.

The hardest moment is the final few aliens. Once most of the formation is destroyed, the remaining aliens move very fast. They cover the screen in seconds. A miss on the last few often means losing a life because the alien has already moved out of your shot's line by the time it gets there.

Lives are limited. Most versions of the format give you three. Each hit destroys one life. Lose all of them and the run ends. The score is tracked across the run.

The visual style is deliberately retro. Pixelated alien sprites, simple bullet shapes, and a flat dark background. The audio uses synthesized chip-tune sounds reminiscent of the original arcade. The whole package fits in a single HTML file, which is part of the charm. There is no install, no signup, no clutter. Just play.

Related: Missile Defense, Bomb Blaster, Brick Buster