This style of game really plays to the strengths of the iphone, and I've been playing a lot of them lately (this, A.S.P., U.S. Forces, Ballistik, and of course Plants vs. Zombies). They are all great games.
There is not all that much strategy, but it has a strange hold over me.
Yeah, the strategy is more in the resource management and in choosing what to upgrade. The gameplay itself is just bouncing back and forth between the catapult, collecting money and dropping footsoldiers. The resource collecting, upgrading and unlocking scratches the same itch as PvZ did but in a game that doesn't feel derivative.