Alien Turrets ©: Mars

Canonically after Alien Turrets 2 & Bot Turrets: Downfall, Aliens are still roaming around the moon until they finally finish building their rocket-ship they've been working on, to go to Mars... Except, you have to chase them in your OWN rocket ship, while flying in space try to shoot them and their laser blasters until you defeat every ship, and at the end, fight the boss's own UFO... It'll be hard.