Well, directional gunfire. Players make their way through a spaceship, destroying all the enemy robots in each room in order to progress. As you kill, money becomes available, which can then be spent on buying extra, more deadly weapons for those slightly harder baddies (especially the ones that fire back).
It’s actually a hell of a lot of fun and definitely worth giving a go – one of the better flash games I’ve played in a good while. You’ll need to pay if you want to play the entire thing though.
Give it a blast on their site.