Krell

Krell is a Space Invaders clone with a twist. You don’t have any barriers
for protection against the enemy formations, instead you have a barrier above
them. This barrier protects seven space aliens from a lone yellow invader who
patrols the upper portion of the screen. Shoot down the invader formation
without hitting the barrier, or you might end up hitting the trapped aliens.
When all the white aliens are dead, it’s game over time, and that is also the
case when you are killed.

Krell