The mission - you are defending the wall from attacking aliens. Don't let them hit as they make the wall weaker. This sounds simplistic and is, but highly addictive as you score points for every alien destroyed. If a segment of the outer wall is destroyed (by the aliens kamikaze into it) if this happens enough times the outer wall flashes and you need to move in a level, post haste. If you don't, you drift off to space and all is lost. Don't fret though you have a chance to regain lost wall space by simply staying alive. If you stay alive long enough with no more incidents, you can regain that which has been lost... precious wall.