There was a point in the early days of the arcades when, all of a sudden, gamers tired of the vertical shooter. Space Invaders had nailed the concept to history's walls, but after a while its clones weren't enough to keep the Japanese coin shortage going.
And so game developers began to get quite imaginative about different ways for players to shoot down enemy ships. With the App Store being as new as it is, all the old concepts could be brought out and paraded once again, but already people are looking for variations. It's to this call that Kryzer Prologue answers.