One button, 5 torpedoes, one target. How hard can it be?
Time your torpedo launch to hit the target in one of its portals to destroy it. Watch out for the defending mini-subs that will try to sacrifice themselves to protect their mothership.
iPad players can opt to play against a friend using two player mode.
Torpedo Alley is also OpenFeint enabled, so compete with other players for top ranking, and collect achievements as you play.