Alien Force

Alien Force places you in control of a space ship on a mission to destroy
all enemy ships. Destroy every ship on a level and you go up to the next level
with more difficult enemies. The action is showed from top-down, and you move
around on a one-screen field with obstacles. You are only allowed to shoot one
missile at a time, so the game is not about non-stop shooting.

In the beginning levels, the enemy ships are easy; not even firing at you. As
you progress through the levels, the ships get harder and start to shoot at
you.

Alien Force