Galaxian is a type of action game where you're at the bottom of the screen, surrounded by aliens above. Your goal is to move left or right to aim at an alien, then shoot a bullet upwards to hit it and kill it. The more powerful aliens are worth more points when destroyed, and they appear in different orders from top to bottom. Every now and then, an enemy will move down towards you, accompanied by a bomb - this is a scary moment.
The gameplay involves controlling the ship at the bottom of the screen to shoot down the incoming aliens and avoid getting hit by the bombs that are dropped by some of them. You'll have to be quick and accurate with your shots to clear each level and move on to the next one, as more enemies will appear as you progress.