Wall-Defender

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, ...